What Causes a Diesel Generator to Fail? 10 Most Common Faults Explained

Most diesel generator failures are not surprises. They are the predictable outcome of an identifiable condition that developed over days, weeks, or months -- and that would have been caught and corrected by a properly executed maintenance programme. Understanding the 10 most common generator fault causes gives operators the knowledge to recognise early warning signs, take corrective action before a fault becomes a failure, and build maintenance programmes that specifically target the highest-risk failure modes.

This guide covers each fault in the same format: what you will see or hear, why it happens, what to do immediately, and how to prevent it from happening again. It applies to all diesel generator sets from 20kW to 2,500kW with Cummins, Perkins, Volvo Penta, or Baudouin engines.

Quick Reference: The 10 Most Common Generator Faults

  Fault 1: Generator Fails to Start

  Severity: High -- facility has no power until fault is resolved   |   How common: Very common -- accounts for approximately 30% of all service calls

  Symptoms -- what you will see or hear

    •  Engine cranks but does not fire

    •  Engine does not crank at all (starter motor silent)

    •  Control panel shows low battery voltage or battery fault alarm

    •  Auto-start attempt fails; panel goes to lockout after 3 crank attempts

  Root causes -- why this happens

    •  Flat or discharged start battery -- the single most common cause; battery cannot deliver cranking current

    •  Low fuel level -- fuel tank run dry; engine cranks but cannot fire

    •  Air in fuel system -- after filter change or tank run dry; engine cranks but does not sustain

    •  Fuel shutoff solenoid stuck closed -- no fuel reaches injection pump

    •  Low temperature -- cold ambient below 5 deg C slows cranking and reduces fuel combustion efficiency

    •  Battery cable corrosion or loose terminal -- high resistance prevents full cranking current

  Immediate action -- what to do right now

    •  Check fuel level first -- if tank empty, refill and bleed fuel system before attempting restart

    •  Check battery voltage on panel display -- if below 10.5V (12V system), battery is discharged

    •  Jump-start from charged battery if available; or connect battery charger and wait 30 minutes

    •  After fuel filter change: bleed fuel system per engine manual before cranking

    •  Check fuel shutoff solenoid: apply 12V or 24V directly to solenoid terminal -- confirm it clicks open

  Prevention -- how to stop it happening again

    •  Weekly battery voltage check -- replace battery at 2-3 year intervals; never wait for failure

    •  Keep fuel tank above 25% at all times -- fuel gauge monitoring with remote alert

    •  Monthly auto-start test -- simulate mains failure and confirm generator starts every time

    •  Annual battery load test or conductance test -- identifies weak batteries before they fail

    •  Trickle charger or battery charger on float mode for standby generators -- maintains battery charge

  Fault 2: Low Oil Pressure Shutdown

  Severity: Critical -- engine can sustain severe damage within seconds if run with low oil pressure   |   How common: Common -- second most frequent cause of emergency shutdown

  Symptoms -- what you will see or hear

    •  Engine shuts down suddenly; control panel shows LOW OIL PRESSURE alarm

    •  Oil pressure gauge drops below minimum (typically 1.5-2.0 bar at idle; 2.5-4.5 bar at load)

    •  Engine may show unusual knocking sound immediately before shutdown if pressure drops gradually

    •  Oil visible under generator base or on engine external surfaces

  Root causes -- why this happens

    •  Low engine oil level -- most common cause; oil consumed or leaked below minimum level

    •  Oil filter blocked -- severely contaminated filter restricts flow to bearings

    •  Oil pressure sensor or switch failure -- spurious signal causes false low-pressure shutdown

    •  Oil pump wear or failure -- reduced pump output insufficient to maintain pressure

    •  Internal oil leakage (cracked oil gallery, worn bearing clearances) -- oil bypasses circuit

    •  Wrong oil viscosity -- oil too thin at operating temperature; insufficient film strength

  Immediate action -- what to do right now

    •  Do not restart the engine until oil level and pressure cause have been investigated

    •  Check oil level immediately -- if low, top up and investigate where oil has gone

    •  Check for external oil leaks -- look under engine, at filter, drain plug, and rocker cover

    •  Check oil pressure sensor reading with handheld gauge -- confirm sensor is reading correctly

    •  If oil level and pressure sensor are normal, suspect internal engine issue -- do not run without specialist inspection

  Prevention -- how to stop it happening again

    •  Daily oil level check -- non-negotiable for prime power generators

    •  Oil and filter change every 250 hours -- degraded oil loses viscosity and film strength

    •  Oil pressure sensor function test at every 500-hour service

    •  Oil analysis every 1,000 hours for high-value prime power generators -- detects wear metals before bearing failure

    •  Check for oil leaks at every weekly inspection -- catch leaks before they deplete oil level

  Fault 3: High Coolant Temperature Shutdown

  Severity: Critical -- sustained overheating warps cylinder head and destroys head gasket   |   How common: Common -- particularly in hot climates and during peak summer ambient temperatures

  Symptoms -- what you will see or hear

    •  Engine shuts down; control panel shows HIGH COOLANT TEMPERATURE alarm

    •  Coolant temperature gauge rising above normal range (typically above 100-105 deg C)

    •  Steam from coolant overflow or visible coolant loss from header tank

    •  Coolant smell from engine compartment

  Root causes -- why this happens

    •  Blocked radiator fins -- dust, debris, or insects blocking airflow through radiator core

    •  Low coolant level -- coolant lost through leak or overflow; air pocket in cooling circuit

    •  Failed cooling fan -- fan belt broken or fan clutch failed; loss of airflow through radiator

    •  Thermostat stuck closed -- coolant cannot circulate to radiator; engine overheats rapidly

    •  Water pump failure -- impeller worn or pump seized; no coolant circulation

    •  Ambient temperature exceeds generator cooling system rating -- common in hot climates above 45 deg C

    •  Generator room ventilation inadequate -- hot air recirculates; effective ambient rises above air temperature

  Immediate action -- what to do right now

    •  Do not remove radiator cap on a hot engine -- severe burn risk from pressurised coolant

    •  Allow engine to cool for 30-60 minutes before inspecting

    •  Check coolant level in header tank after cooling -- top up with correct coolant mixture if low

    •  Inspect radiator fins -- if blocked with dust or debris, clean with compressed air or low-pressure water

    •  Check fan belt -- if broken or slipping, replace before restarting

    •  Check generator room ventilation -- confirm hot air exhaust is not blocked or recirculating

  Prevention -- how to stop it happening again

    •  Monthly radiator fin inspection and cleaning -- critical in dusty environments

    •  Coolant level check at every daily inspection

    •  Thermostat replacement at 1,000-hour annual service

    •  Generator room ventilation design -- confirm adequate airflow at maximum ambient temperature

    •  For hot climates: confirm generator cooling system is rated for local ambient + 5 deg C margin

  Fault 4: Fuel Starvation -- Engine Hunting or Loss of Power

  Severity: High -- generator cannot maintain rated output; frequency and voltage become unstable   |   How common: Very common -- particularly after maintenance or in markets with poor fuel quality

  Symptoms -- what you will see or hear

    •  Engine speed oscillates (hunting) -- frequency gauge swings above and below rated value

    •  Engine losing power progressively -- generator cannot maintain rated load

    •  Engine stalls under load, especially when large loads are applied

    •  Black smoke at exhaust (fuel-lean combustion producing incomplete burn)

    •  Engine starts but shuts down within 30-60 seconds of running

  Root causes -- why this happens

    •  Blocked fuel filter -- most common cause after filter is overdue for replacement or fuel quality is poor

    •  Air in fuel system -- after filter change, tank run-dry, or cracked fuel line; air breaks injection pattern

    •  Water in fuel -- water reaching injectors causes misfiring and speed instability

    •  Fuel lift pump failure -- insufficient fuel pressure reaching injection pump

    •  Blocked fuel tank strainer or tank vent -- creating vacuum that restricts fuel flow

    •  Injector nozzle wear or blockage -- reduced or distorted spray pattern; poor atomisation

  Immediate action -- what to do right now

    •  Check fuel level first -- run-dry is the simplest explanation for fuel starvation

    •  Inspect primary fuel filter bowl for water or sediment -- drain if contaminated

    •  If filter was recently changed: bleed fuel system to remove trapped air per engine manual

    •  Check fuel lines from tank to engine for kinks, blockage, or signs of collapse

    •  Operate primer pump (if fitted) manually to build fuel pressure before cranking

  Prevention -- how to stop it happening again

    •  Replace fuel filter at every 250-hour service -- do not extend intervals in dirty fuel markets

    •  Bleed fuel system correctly after every filter change -- never skip this step

    •  Monthly water check in fuel tank -- drain free water before it reaches filters

    •  Fuel quality management -- stabiliser and biocide at delivery; test fuel every 6 months

    •  Injector bench test at 1,000-hour annual service

  Fault 5: Overload Trip / Low Voltage Under Load

  Severity: Medium -- generator trips; facility loses power until load is reduced and generator restarted   |   How common: Common -- particularly when facilities add equipment without reviewing generator capacity

  Symptoms -- what you will see or hear

    •  Generator trips on overload protection; control panel shows OVERLOAD alarm

    •  Output voltage drops below normal when large loads are connected

    •  Generator engine struggles under load -- speed drops; frequency falls below rated value

    •  Circuit breakers in distribution board trip repeatedly when generator is supplying power

  Root causes -- why this happens

    •  Total connected load exceeds generator rated output -- most common cause

    •  Large motor starting surge temporarily exceeds generator capacity -- voltage collapses during start

    •  Power factor of load lower than generator rated power factor (0.8) -- alternator reaches kVA limit before kW limit

    •  Generator is derated for altitude or temperature but operating at nameplate without correction

    •  Faulty overload protection setting -- protection trips below actual generator capability

  Immediate action -- what to do right now

    •  Shed non-essential loads -- switch off equipment that is not critical to operation

    •  Reset overload protection on control panel after reducing load

    •  Restart generator and reconnect essential loads only

    •  For motor starting trips: start large motors one at a time with all other loads stable

    •  Measure actual load (kW and kVA) with a clamp meter -- confirm whether load exceeds generator rating

  Prevention -- how to stop it happening again

    •  Load audit before connecting new equipment -- measure existing load, calculate headroom

    •  Soft-starters or VFDs on large motors -- reduces starting surge from 500% to 150% of running current

    •  Generator sizing review when facility load grows -- do not assume existing unit is adequate

    •  kVA vs kW awareness -- confirm power factor of connected loads against generator rated power factor

  Fault 6: Alternator Excitation Failure -- No Voltage Output

  Severity: High -- generator runs mechanically but produces no electrical output   |   How common: Moderate -- but often misdiagnosed as an engine problem

  Symptoms -- what you will see or hear

    •  Generator engine runs at correct speed and frequency but output voltage is zero or very low

    •  No load can be connected -- voltage collapses immediately on any load application

    •  Control panel shows under-voltage alarm with engine running normally

    •  AVR fault light or LED on voltage regulator board

  Root causes -- why this happens

    •  AVR (Automatic Voltage Regulator) failure -- most common cause; AVR board fails or loses control signal

    •  Loss of residual magnetism -- alternator rotor loses residual magnetic field; cannot self-excite

    •  Excitation winding open circuit -- broken connection in rotor excitation circuit

    •  Brushes worn to minimum length (brush-type alternators) -- no contact with slip rings

    •  Slip ring surface oxidised or contaminated -- high resistance prevents excitation current flow

    •  Main stator winding fault -- shorted or open-circuit winding reduces voltage generation

  Immediate action -- what to do right now

    •  Flash the alternator (restore residual magnetism): connect a 12V battery briefly to the excitation winding terminals -- restores residual field and allows self-excitation. Procedure varies by alternator type -- refer to alternator manual.

    •  Check AVR board for burnt components, blown fuses, or damaged potentiometer

    •  Measure excitation voltage at AVR output terminals -- if zero, AVR has failed; replace AVR board

    •  Inspect brushes and slip rings (if brush-type alternator) -- clean slip rings; replace brushes if worn to minimum

    •  Megger test stator windings -- if insulation resistance is low, winding fault is present

  Prevention -- how to stop it happening again

    •  Annual AVR inspection -- check connections, components, and voltage set point calibration

    •  Annual insulation resistance test -- catches winding degradation before complete failure

    •  Brush inspection at 500-hour service (brush-type alternators) -- replace before worn to minimum

    •  Alternator kept clean and dry -- moisture is the primary cause of insulation degradation

    •  For long-idle standby generators: run under load monthly to maintain residual magnetism

  Fault 7: Excessive Exhaust Smoke and High Exhaust Temperature

  Severity: Medium-high -- indicates combustion problem; power loss and accelerated engine wear   |   How common: Common -- particularly in generators with overdue air filter or injector service

  Symptoms -- what you will see or hear

    •  Black smoke: fuel-rich combustion -- insufficient air or excess fuel

    •  Blue smoke: oil burning -- oil entering combustion from worn rings or valve seals

    •  White smoke: unburned fuel or coolant -- cold start (normal) or head gasket failure (serious)

    •  Power output reduced; engine struggles to maintain rated load

    •  Exhaust temperature above normal -- exhaust gas temperature gauge reading high

  Root causes -- why this happens

    •  Black smoke -- blocked air filter restricting airflow; turbocharger not producing rated boost; injector delivering excess fuel

    •  Blue smoke -- piston ring wear allowing oil past rings; worn valve stem seals; overfilling engine oil

    •  White smoke (persistent, not just cold start) -- coolant entering combustion via cracked head or failed head gasket

    •  All smoke types -- injector nozzle wear causing poor fuel atomisation and incomplete combustion

  Immediate action -- what to do right now

    •  Black smoke: inspect air filter immediately -- if blocked, replace before further operation

    •  Black smoke persistent after new air filter: suspect turbocharger or injector issue -- reduce load and arrange inspection

    •  Blue smoke: check oil level -- if overfull, drain to correct level; if correct, suspect ring or valve seal wear

    •  White smoke (persistent): immediately check coolant level and oil for milky contamination -- if coolant in oil, shut down; head gasket failure suspected

    •  Reduce load to minimum and run at no-load -- if smoke clears, load was excessive for engine condition

  Prevention -- how to stop it happening again

    •  Air filter replacement at every 250-hour service -- extended intervals cause restriction and black smoke

    •  Annual injector bench test -- worn nozzles cause black smoke and power loss

    •  Avoid sustained low-load operation (wet stacking -- see Fault 10)

    •  Annual compression test -- declining compression indicates ring wear before blue smoke appears

    •  Coolant condition check at every 500-hour service -- contaminated coolant indicates developing head gasket issue

  Fault 8: Battery and Starting System Failure

  Severity: High -- generator cannot start on demand; no backup power when grid fails   |   How common: Common -- battery failure is one of the most preventable generator problems

  Symptoms -- what you will see or hear

    •  Generator does not respond to auto-start signal -- no cranking

    •  Slow cranking -- engine turns over slowly and does not fire; battery voltage drops rapidly under cranking load

    •  Clicks but no crank -- relay clicking but solenoid cannot engage; battery cannot deliver cranking current

    •  Battery warning alarm on control panel; battery voltage below 11V (12V system) or 22V (24V system)

  Root causes -- why this happens

    •  Battery end of service life -- internal cell degradation reduces capacity; most common cause

    •  Battery discharged -- no float charger fitted; battery slowly self-discharges between starts

    •  Battery charger failure -- charger circuit fault; battery not being maintained at float charge

    •  Corroded battery terminals -- high resistance connection prevents full current delivery

    •  Faulty starter motor solenoid -- mechanical or electrical failure; cannot engage starter

    •  Undersized battery for engine cranking requirement -- original battery replaced with lower capacity unit

  Immediate action -- what to do right now

    •  Check battery terminal voltage -- below 11V (12V system) means battery is discharged or failed

    •  Jump-start from an external charged battery or battery pack to confirm starter motor is functional

    •  If engine starts from jump -- battery has failed or is discharged; charge and load-test before relying on it

    •  Clean corroded terminals with wire brush and sodium bicarbonate solution; apply petroleum jelly after cleaning

    •  Check battery charger output -- should be 13.5-14.4V (12V system) in float mode

  Prevention -- how to stop it happening again

    •  Float charger (trickle charger) on all standby generators -- maintains battery at full charge at all times

    •  Battery replacement every 2-3 years regardless of apparent condition -- the cost of a battery is trivial compared to the cost of a failed start during a power outage

    •  Annual battery load test or conductance test at 1,000-hour service

    •  Dual battery system for critical applications -- independent primary and standby battery banks

    •  Monthly auto-start test -- confirms battery can deliver cranking current under real conditions

  Fault 9: Control Panel Fault / Generator Locked Out on Alarm

  Severity: Medium -- generator disabled; requires fault diagnosis to restore operation   |   How common: Moderate -- often caused by sensor faults rather than actual engine problems

  Symptoms -- what you will see or hear

    •  Generator shut down and locked out; control panel shows a fault code or alarm

    •  Fault alarm present but engine parameters appear normal when checked manually

    •  Generator starts and then immediately shuts down on a sensor-triggered alarm

    •  Intermittent alarms that appear and disappear without apparent cause

  Root causes -- why this happens

    •  Faulty sensor -- oil pressure sensor, temperature sensor, or speed sensor giving spurious reading

    •  Loose or corroded sensor wiring connection -- intermittent signal causes false alarm

    •  Control panel software fault or corrupted parameter -- incorrect alarm setpoint

    •  Genuine engine fault masked by control panel confusion -- fault code does not clearly identify root cause

    •  Incorrect panel settings after a service or parameter reset -- protection thresholds set incorrectly

  Immediate action -- what to do right now

    •  Read and record all fault codes from the control panel -- DSE and ComAp panels display fault history

    •  Cross-reference fault code with actual engine parameters -- measure oil pressure, temperature with separate instruments

    •  If actual parameters are normal, suspect sensor or wiring fault -- inspect sensor connector and wiring

    •  Reset lockout after resolving genuine fault -- do not reset without understanding what caused the lockout

    •  If fault code indicates genuine engine problem (confirmed by independent measurement) -- do not override protection

  Prevention -- how to stop it happening again

    •  Sensor wiring inspection at every 500-hour service -- check all sensor connectors for corrosion and security

    •  Control panel parameter backup -- record all parameter settings after commissioning; restore after any panel reset

    •  Annual protection system test -- verify each protection input (oil pressure switch, temp sensor) functions correctly

    •  Conformal coat on control panel PCBs for tropical and coastal installations -- prevents corrosion of board components

    •  DSE panel fault log download at every service -- reveals intermittent faults that do not result in visible alarms

  Fault 10: Wet Stacking -- Sustained Low-Load Operation

  Severity: Medium -- progressive engine damage; reduced engine life if not corrected   |   How common: Common in standby generators that run at light load during testing or infrequent grid failures

  Symptoms -- what you will see or hear

    •  Black or dark grey oily residue around exhaust pipe outlet and silencer joints

    •  Oily deposits visible inside exhaust pipe when inspected

    •  Blue-grey haze in exhaust at light load

    •  Progressive power loss over time; engine cannot maintain rated output

    •  Engine oil becoming diluted -- diesel mixing into sump oil via cylinder walls

  Root causes -- why this happens

    •  Generator running consistently below 30% of rated load -- insufficient combustion temperature to fully burn fuel

    •  At low load, exhaust gas temperature drops below the temperature needed to vaporise unburned fuel and oil

    •  Unburned fuel condenses in exhaust system as liquid -- mixes with oil vapour to form the wet stack deposits

    •  Standby generators tested at no-load or very light load only -- never reach operating temperature

    •  Hybrid system generators running as backup only -- insufficient load hours at adequate load level

  Immediate action -- what to do right now

    •  Load bank test -- connect a load bank at 75-100% of rated generator output for a minimum of 2 hours

    •  This burns off accumulated deposits in the exhaust and combustion system

    •  After load bank test: inspect exhaust for reduced deposits; check engine oil for fuel dilution (oil level will rise if diesel is mixing into sump)

    •  If engine oil is fuel-diluted: change oil and filter immediately after the load bank run

    •  Reduce ambient light load operation -- do not run standby generators at low load for extended periods

  Prevention -- how to stop it happening again

    •  Monthly load test at minimum 50% rated load for at least 30 minutes -- sufficient to reach operating temperature and burn deposits

    •  Weekly test run for standby generators in markets with infrequent grid outages

    •  Correctly size generator to site load -- an oversized generator will always operate at low load percentage

    •  In hybrid systems: configure generator minimum load threshold at 40-50% of rated output

    •  If site load is genuinely low: specify a smaller generator correctly sized to site demand

Summary: The Maintenance Actions That Prevent 80% of All Faults

  Eight of the ten faults above are prevented by a single well-executed preventive maintenance programme. The two exceptions -- overload trip (Fault 5) and wet stacking (Fault 10) -- are operational decisions, not maintenance failures. Fix those two with correct generator sizing and regular loaded testing. Fix the other eight with the maintenance schedule in our Generator Maintenance Checklist guide.

Leading Power Technical Support

If your Leading Power generator has developed a fault that is not resolved by the steps above, our engineering team provides direct technical support. Send us the fault code, the engine model, and a description of the operating conditions when the fault occurred. We respond within 24 hours with a diagnosis and recommended corrective action.

· Fault code library: DSE and ComAp fault codes explained for all Cummins, Perkins, Volvo Penta, and Baudouin engine configurations

· Remote diagnosis: our engineers can connect to DSE 8610 or ComAp InteliGen panels via Modbus/Ethernet for remote fault reading

· Service contractor network: authorised service contractors available in Nigeria, Ghana, Kenya, Tanzania, Saudi Arabia, UAE, and Indonesia

· Spare parts supply: genuine Cummins, Perkins, and Stamford parts available for all Leading Power generator configurations

· Warranty support: 12-month warranty covers manufacturing defects; engine OEM warranty covers engine-specific failures

· 24-hour technical response -- send fault code and engine model for immediate diagnosis support

Leading Power is a CE-certified diesel generator manufacturer based in Fu'an, Fujian, China. Established in 2008. 5kW-3,000kW generator sets. Cummins, Perkins, Volvo Penta, and Baudouin engines. 24-hour technical support for all Leading Power generators. Service contractor network in 20+ countries.