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Cummins/Onan Spec C Troubleshooting

Cummins/Onan Spec C troubleshooting: 7 fault codes and 207 symptom-cause-remedy entries from the manufacturer's own service documentation.

Cummins/Onan Spec C — full specs, parts & maintenance.

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Troubleshooting

214

Fault codes

Code	Meaning	Cause	Remedy
F11	Engine does not crank	Control fuse F11 open	Replace fuse (10 ampere).
CB12	Engine starts but stops when start switch is released	Fault breaker CB12 not completing circuit due to defective breaker	Check continuity through breaker and replace if defective.
CB12	Engine starts but stops when start switch is released	Fault breaker CB12 not completing circuit due to fault condition	Check oil level, coolant level, and sea water cooling system. See applicable section in Primary Systems.
CB12	Engine starts but stops when start switch is released	Fault breaker CB12 not completing circuit due to defective sensor	Check sensors per Primary Systems checkout procedures.
CB12	Engine starts but stops when start switch is released	Fault breaker CB12 not completing circuit due to open circuit	Check wiring for open or loose connections.
CB12	Engine starts but stops when start switch is released	Fault breaker CB12 not completing circuit due to defective A11-K12 relay	Replace PC board A11.
CR2	Low battery, generator set does not recharge	Defective CR2 bridge rectifier	Test CR2 bridge and replace if defective.

Symptoms & remedies

Symptom	Cause	Remedy
Engine cranks too stiffly	Too heavy oil in crankcase	Drain. Refill with light oil. See PREPARATION.
Engine cranks too stiffly	Engine stuck	Disassemble and repair.
Engine cranks too slowly when cranked electrically	Discharged or defective battery	Recharge or replace.
Engine cranks too slowly when cranked electrically	Loose connections	Tighten loose connections.
Engine cranks too slowly when cranked electrically	Corroded battery terminals	Clean corroded terminals. Replace cable if necessary.
Engine cranks too slowly when cranked electrically	Brushes worn excessively or making poor contact	Replace brushes or clean commutator.
Engine cranks too slowly when cranked electrically	Short circuit in generator load circuit	Repair or replace parts necessary. Disconnect load.
Engine cranks too slowly when cranked electrically	Dirty or corroded points in start solenoid switch	Replace switch.
Engine will not start when cranked	Faulty ignition	Clean, adjust, or replace breaker points, spark plugs, condenser, etc., or retime ignition.
Engine will not start when cranked	Lack of fuel or faulty carburetion	Refill the tank. Check the fuel system. Clean, adjust, or replace parts necessary.
Engine will not start when cranked	Cylinders flooded	Ground spark plug cables. Crank engine with spark plugs removed.
Engine will not start when cranked	Poor fuel	Drain. Refill with good fuel.
Engine will not start when cranked	Poor compression	Tighten cylinder heads and spark plugs. If still not corrected, grind the valves. Replace piston rings if necessary.
Engine will not start when cranked	Wrong ignition timing	Reset breaker points or retime ignition. See IGNITION TIMING.
Engine runs but voltage does not build up	Poor brush contact	See that brushes seat well on commutator and collector rings, are free in holders, are not worn shorter than 1/2 inch, and have good spring tension.
Engine runs but voltage does not build up	Open circuit, short circuit, or ground in generator	Refer to the GENERATOR section of Maintenance.
Voltage unsteady but engine not misfiring	Speed too low	Adjust governor to correct speed.
Voltage unsteady but engine not misfiring	Poor commutation or brush contact	Refinish commutator or undercut mica if necessary. See that brushes seat well on commutator and collector rings, are free in holders, are not worn shorter than 1/2 inch, and have good spring tension.
Voltage unsteady but engine not misfiring	Loose connections	Tighten connections.
Voltage unsteady but engine not misfiring	Fluctuating load	Correct any abnormal load condition causing trouble.
Generator overheating	Short in load circuit	Correct short circuit.
Generator overheating	Generator overloaded	Reduce the load.
Generator overheating	Improper brush rig position	Refer to the GENERATOR section of MAINTENANCE - See Brush Rig.
Engine overheating	Improper lubrication	See Low Oil Pressure.
Engine overheating	Poor ventilation	Provide ample ventilation at all times.
Engine overheating	Dirty or oily cooling surfaces	Keep the engine clean.
Engine overheating	Retarded ignition timing	Retime ignition.
Engine overheating	Generator overloaded	Reduce load.
Voltage drops under heavy load	Engine lacks power	See remedies under "Engine Misfires at Heavy Load".
Voltage drops under heavy load	Poor compression	Tighten cylinder heads and spark plugs. If still not corrected, grind the valves. Replace piston rings if necessary.
Voltage drops under heavy load	Faulty carburetion	Check the fuel system. Clean, adjust or repair as needed.
Voltage drops under heavy load	Dirty carburetor air cleaner	Clean and Service.
Voltage drops under heavy load	Choke partially closed	Choke plate must be wide open at operating temperature.
Voltage drops under heavy load	Carbon in cylinders or in carburetor venturi	Remove carbon.
Voltage drops under heavy load	Restricted exhaust line	Clean or increase the size.
Voltage drops under heavy load	Improper governor adjustment	Refer to ADJUSTMENTS.
Engine misfires at light load	Carburetor idle jet clogged or improperly adjusted	Clean or adjust.
Engine misfires at light load	Spark plug gaps too narrow	Adjust to correct gap - .025" (.018" for gas operation).
Engine misfires at light load	Intake air leak	Tighten manifold and carburetor mounting screws. Replace gaskets if necessary.
Engine misfires at light load	Faulty ignition	Clean, adjust, or replace breaker points, spark plugs, condenser, etc.
Engine misfires at heavy load	Defective spark plug	Replace.
Engine misfires at heavy load	Faulty ignition	Clean, adjust, or replace breaker points, spark plugs, condensers, etc. or retime ignition.
Engine misfires at heavy load	Clogged carburetor	Clean carburetor.
Engine misfires at heavy load	Clogged fuel screen	Clean.
Engine misfires at heavy load	Defective spark plug cable	Replace.
Engine misfires at all loads	Fouled spark plug	Clean and adjust.
Engine misfires at all loads	Defective or wrong spark plug	Replace.
Engine misfires at all loads	Leaking valves	See VALVE SERVICE.
Engine misfires at all loads	Broken valve spring	Replace.
Engine misfires at all loads	Defective or improperly adjusted breaker points	Adjust or replace breaker points.
Low oil pressure	Oil too light	Drain, refill with proper oil.
Low oil pressure	Oil badly diluted	Drain, refill with proper oil.
Low oil pressure	Oil too low	Add oil.
Low oil pressure	Oil relief valve not seating	Remove and clean, or replace.
Low oil pressure	Badly worn bearings	Replace.
Low oil pressure	Sludge on oil screen	Remove and clean.
Low oil pressure	Badly worn oil pump	Replace.
Low oil pressure	Defective oil pressure gauge	Replace.
High oil pressure	Oil too heavy	Drain, refill with proper oil.
High oil pressure	Clogged oil passage	Clean all lines and passages.
High oil pressure	Oil relief valve stuck	Remove and clean.
High oil pressure	Defective oil pressure gauge	Replace.
Engine backfires	Lean fuel mixture	Clean carburetor. Adjust jets.
Engine backfires	Clogged fuel filter	Clean.
Engine backfires	Air leak at intake manifold or carburetor flange	Tighten mounting screws. Replace gaskets if necessary.
Engine backfires	Poor fuel	Refill with good, fresh fuel. See PREPARATION.
Engine backfires	Spark advanced too far	Reset breaker points or retime ignition.
Engine backfires	Intake valve leaking	Reseat or replace.
Excessive oil consumption, light blue exhaust	Poor compression. Usually due to worn pistons, rings, or cylinders	Refinish cylinders. Install oversize pistons and rings.
Excessive oil consumption, light blue exhaust	Oil too light or diluted	Drain. Refill with proper oil.
Excessive oil consumption, light blue exhaust	Too large bearing clearance	Replace bearings necessary.
Excessive oil consumption, light blue exhaust	Engine misfires	Refer to "Engine Misfires At All Loads".
Excessive oil consumption, light blue exhaust	Faulty ignition	Clean, adjust, or replace breaker points, spark plugs, condenser, etc., or retime the ignition.
Excessive oil consumption, light blue exhaust	Too much oil	Drain excess oil.
Black, smoky exhaust, excessive fuel consumption, fouling of spark plugs with black soot, possible lack of power under heavy load	Fuel mixture too rich	See that choke opens properly. Adjust jets properly. Adjust the float level.
Black, smoky exhaust, excessive fuel consumption, fouling of spark plugs with black soot, possible lack of power under heavy load	Choke not fully open	See that choke opens properly.
Black, smoky exhaust, excessive fuel consumption, fouling of spark plugs with black soot, possible lack of power under heavy load	Dirty air cleaner	Clean and Service.
Light pounding knock	Loose connecting rod	Replace rod bearings.
Light pounding knock	Low oil supply	Add oil. Change if necessary.
Light pounding knock	Oil badly diluted	Drain. Refill with proper oil.
Light pounding knock	Low oil pressure	See Low Oil Pressure for remedies.
Engine stops unexpectedly	Empty fuel tank	Refill.
Engine stops unexpectedly	Defective ignition system	Check the ignition system. Repair or replace as needed. See that the STOP button lead is not grounded.
Engine stops unexpectedly	Fuel pump failure	Repair or replace.
Dull metallic thud, if not bad, may disappear after few minutes operation, if bad, increases with load	Loose crankshaft bearing	Replace, unless one of the next two remedies permanently corrects the trouble.
Sharp metallic thud, especially when cold engine first started	Low oil supply	Add oil. Change if necessary.
Sharp metallic thud, especially when cold engine first started	Oil badly diluted	Drain. Refill with proper oil.
Pinging sound when engine is suddenly or heavily loaded	Carbon in cylinders	Remove the carbon.
Pinging sound when engine is suddenly or heavily loaded	Spark advanced too far	Reset breaker points or retime ignition.
Pinging sound when engine is suddenly or heavily loaded	Wrong spark plugs	Install correct spark plugs.
Pinging sound when engine is suddenly or heavily loaded	Spark plugs burned or carboned	Clean. Install new plugs if necessary.
Pinging sound when engine is suddenly or heavily loaded	Valves hot	Adjust tappet clearance. See VALVE SERVICE.
Pinging sound when engine is suddenly or heavily loaded	Fuel stale or low octane	Use good, fresh fuel. See PREPARATION.
Pinging sound when engine is suddenly or heavily loaded	Lean fuel mixture	Clean fuel system. Adjust carburetor jets properly.
Tapping sound	Valve clearance too great	Adjust to proper clearance. See VALVE TAPPETS.
Tapping sound	Broken valve spring	Install new spring.
Hollow clicking sound with cool engine under load	Loose piston	If noise is only slight and disappears when engine warms up, no immediate attention needed. Otherwise replace parts necessary.
Sharp click when cranking engine	Magneto impulse coupling	Normal condition - should stop as soon as engine starts.
Voltage low at far end of line but normal near plant	Too small line wire used for load and distance	Install larger or extra wires or reduce load.
Motors run too slowly and overheat at far end of line but OK near the plant	Too small line wire used for load and distance	Install larger or extra wires or reduce load.
Excessive arcing of brushes	Rough commutator or rings	Turn down.
Excessive arcing of brushes	Dirty commutator or rings	Clean.
Excessive arcing of brushes	Brushes not seating properly	Sand to a good seat or reduce load until worn in.
Excessive arcing of brushes	Open circuit in armature	Install a new armature.
Excessive arcing of brushes	Brush rig out of position	Line up properly.
Spark plugs foul up rapidly	Engine running "cold"	Restrict air flow. Install preheater hose.
Spark plugs foul up rapidly	Wrong plugs	Replace with correct plugs.
Spark plugs foul up rapidly	Carburetor too "rich"	Adjust.
Oil dilution	One spark plug fouled	Clean plugs.
Oil dilution	Leaky carburetor valve	Clean.
Oil seal leak	Worn oil seals	Replace.
Oil seal leak	Fouled breather valve	Clean or replace.
Oil seal leak	Loose oil fill cap	Tighten - replace if gasket is damaged.
Engine misfires	Faulty ignition due to worn or fouled spark plugs	Replace spark plugs.
Engine misfires	Faulty ignition due to incorrect ignition timing	Set distributor.
Engine misfires	Faulty ignition due to faulty ignition coil	Test coil and replace if necessary.
Engine misfires	Faulty ignition due to faulty plug wires	Test spark plug wires and replace if faulty.
Engine misfires	Lean fuel mixture due to incorrectly adjusted fuel mixture screws	Adjust carburetor main and idle adjustment screws.
Engine misfires	Lean fuel mixture due to incorrect float level	Adjust carburetor float level.
Engine misfires	Lean fuel mixture due to dirt in carburetor	Disassemble carburetor and clean all internal passages.
Engine misfires	Lean fuel mixture due to vacuum leak	Locate leak and correct as required.
Engine misfires	Contaminated fuel	Drain fuel tank and refill with fresh fuel.
Engine backfires	Faulty ignition due to incorrect ignition timing	Adjust distributor.
Engine backfires	Faulty ignition due to incorrect spark plug gap	Reset spark plug gap.
Engine backfires	Lean fuel mixture due to incorrectly adjusted fuel mixture screws	Adjust carburetor main and idle adjustment screws.
Engine backfires	Lean fuel mixture due to incorrect float level	Adjust carburetor float level.
Engine backfires	Lean fuel mixture due to dirt in carburetor	Disassemble carburetor and clean all internal passages.
Engine backfires	Mechanical damage to engine	See Section 9 - Engine Block Assembly section.
Engine lacks power	Faulty ignition due to incorrect ignition timing	Adjust distributor.
Engine lacks power	Faulty ignition due to incorrect spark plug gap	Reset spark plug gap.
Engine lacks power	Restricted fuel flow due to faulty fuel pump	Test fuel pump and replace if faulty.
Engine lacks power	Incorrect fuel mixture due to incorrectly adjusted fuel mixture screws	Adjust carburetor main and idle adjustment screws.
Engine lacks power	Incorrect fuel mixture due to incorrect float level	Adjust carburetor float level.
Engine lacks power	Incorrect fuel mixture due to dirt in carburetor	Disassemble carburetor and clean all internal passages.
Engine lacks power	Exhaust system blocked or restricted	Locate and remove cause of blockage.
Engine lacks power	Incorrect valve clearance	Adjust valve clearance (see Section 9 - Engine Block Assembly).
Engine lacks power	Excessive engine wear or damage to engine	See Section 9 - Engine Block Assembly.
Engine overheats	Restricted airflow due to dirt or debris blocking air inlet or outlet	Clear away any debris that may restrict airflow to set. Do not use compartment for storage area.
Engine overheats	Dirt or oil covering engine cooling tins	Clean away all dirt and oil from engine cooling tins.
Engine overheats	Incorrect ignition timing	Adjust distributor.
Engine overheats	Lean fuel mixture due to incorrectly adjusted fuel mixture screws	Adjust carburetor main and idle adjustment screws.
Engine overheats	Lean fuel mixture due to incorrect float level	Adjust carburetor float level.
Engine overheats	Lean fuel mixture due to dirt in carburetor	Disassemble carburetor and clean all internal passages.
Engine overheats	Defective sea water pump	Check sea water flow rate. If low, pump impeller may be bad.
Black exhaust	Rich fuel mixture due to choke sticking	Clean choke and choke linkage.
Black exhaust	Rich fuel mixture due to incorrectly adjusted fuel	Adjust carburetor idle and main adjustment screws.
Black exhaust	Rich fuel mixture due to dirt in carburetor	Disassemble carburetor and clean all internal passages.
Black exhaust	Rich fuel mixture due to misadjusted choke	Adjust to spec.
Black exhaust	Rich fuel mixture due to leaking pull-off diaphragm	Test and replace if defective.
Black exhaust	Rich fuel mixture due to choke heater open	Replace if open.
Black exhaust	Rich fuel mixture due to choke bi-metal broken	Replace if broken.
Engine hunts or surges	Sticking or binding governor linkage	Clean governor linkage.
Engine hunts or surges	Incorrect governor adjustment	Adjust governor speed and sensitivity.
Engine hunts or surges	Faulty governor spring (prior to Spec C gensets only)	Replace governor spring.
Engine hunts or surges	Incorrect fuel mixture due to incorrectly adjusted fuel mixture screws	Adjust carburetor main and idle adjustment screws.
Engine hunts or surges	Incorrect fuel mixture due to incorrect float level	Adjust carburetor float level.
Engine hunts or surges	Incorrect fuel mixture due to dirt in carburetor	Disassemble carburetor and clean all internal passages.
High oil consumption (Note: New engines sometimes have high oil consumption during break-in)	Oil viscosity too light or oil is diluted	Drain oil and refill with correct viscosity oil.
High oil consumption (Note: New engines sometimes have high oil consumption during break-in)	Crankcase breather valve is dirty or defective	Clean crankcase breather and replace if defective.
High oil consumption (Note: New engines sometimes have high oil consumption during break-in)	Oil leaks	Locate source of leak and repair as required.
High oil consumption (Note: New engines sometimes have high oil consumption during break-in)	Excessive engine wear	See Section 9 - Engine Block Assembly.
High oil consumption (Note: New engines sometimes have high oil consumption during break-in)	Light loading	Don't run set at no load for long periods of time.
Low oil pressure	Oil viscosity too light or oil is diluted	Drain oil and refill with correct viscosity oil.
Low oil pressure	Low oil level	Add oil as required.
Low oil pressure	Low oil pressure switch defective	Replace oil pressure switch (see Section 9 - Engine Block Assembly).
Low oil pressure	Faulty oil bypass valve	Inspect oil bypass valve and clean or replace as required (see Section 9 Engine Block Assembly).
Low oil pressure	Excessive engine wear or defective oil pump	See Section 9 - Engine Block Assembly.
Engine does not crank	Insufficient voltage for cranking due to terminal connection loose or corroded	Clean and tighten connections at battery, K11 start solenoid, and the starter motor.
Engine does not crank	Insufficient voltage for cranking due to battery dead or not charged	Check condition of battery and recharge or replace.
Engine does not crank	If engine cranks at set but not at the remote control panel, cause may be open circuit in remote control	Check for continuity and correct if circuit is open.
Engine does not crank	If engine cranks at set but not at the remote control panel, cause may be fault with remote start switch	Replace remote start switch.
Engine does not crank	If engine cranks at remote control panel but not at set, cause may be open circuit in wiring between S11 switch and engine monitor board A11	Check for continuity and correct if circuit is open.
Engine does not crank	If engine cranks at remote control panel but not at set, cause may be fault in S11 switch	Replace S11 switch.
Engine does not crank	With battery voltage present at terminal S on the start solenoid with S11 in Start position, fault may be defective K11 solenoid	Replace K11 start solenoid.
Engine does not crank	If voltage is not present as described in step 5, cause may be open circuit between K11 solenoid and engine monitor PC board A11	Check for continuity and correct if circuit is open.
Engine does not crank	If voltage is not present as described in step 5, cause may be defective PC board A11	Replace PC board A11.
Engine cranks but does not start	Faulty ignition due to defective spark plugs, plug wires, coil, electronic module, or incorrect ignition timing	Refer to Ignition System section for test and service procedures.
Engine cranks but does not start	Faulty fuel system due to sticking choke, faulty fuel pump, plugged filter, or carburetor misadjusted	Refer to Fuel System section for test and service procedures.
Engine cranks but does not start	If battery voltage is not present at positive (+) terminal on coil with S11 in the Start position, fault may be open circuit between coil and control	Check for continuity and correct if circuit is open.
Engine cranks but does not start	Defective K14 ignition start relay (starting Spec C only)	Replace relay K14.
Engine starts but stops when start switch is released (continued)	AC voltage (min 90 VAC) from generator leads 1 and 2 not being supplied to AC terminals of PC board A11 due to defective flash diode CR1	Test diode CR1 for open condition and replace if defective.
Engine starts but stops when start switch is released (continued)	If voltage is present as specified in step 2, cause may be monitor board A11	Replace monitor board A11.
Low battery, generator set does not recharge	Weak or discharged battery	Connect a separate battery charger to bring battery up to full charge. If battery will not take charge, replace it.
Low battery, generator set does not recharge	Load connected to battery while set is turned off	Turn off load.
Low battery, generator set does not recharge	Generator set not run long enough for built-in charger to recharge battery	Supplement charging with a battery charger when boat is at dockside.
Low battery, generator set does not recharge	Defective stator winding (B1-B2)	Refer to Section 8 - Generator for testing and service.
Engine overspeeds, or speed is erratic	Defective or loose belts (prior to Spec C)	Replace defective belts or adjust tension.
Engine overspeeds, or speed is erratic	Defective component in electronic governing system (starting Spec C)	See Governor service procedures in Section 6.
No AC Output Voltage — This condition may cause the generator set to stop when the Start switch is released	Open circuit breaker	Locate cause of overload and correct as required; Reset breaker
No AC Output Voltage — This condition may cause the generator set to stop when the Start switch is released	Open circuit between brush block and CR21 rectifier	Check for continuity and correct if circuit is open
No AC Output Voltage — This condition may cause the generator set to stop when the Start switch is released	Brushes stuck in holder or not making good contact with slip rings	Release brushes if jammed in holder; Clean slip rings if dirty
No AC Output Voltage — This condition may cause the generator set to stop when the Start switch is released	Defective CR21 bridge rectifier	Test rectifier and replace if defective
No AC Output Voltage — This condition may cause the generator set to stop when the Start switch is released	Open, grounded, or short circuit in rotor, stator, or transformer	Test each component for open, grounded, or shorted windings and replace if defective
AC Output Voltage Too Low Or Too High	Engine governor incorrectly adjusted	Refer to Governor section
AC Output Voltage Too Low Or Too High	Open diode in CR21 bridge rectifier	Test CR21 rectifier and replace if defective
AC Output Voltage Too Low Or Too High	Brushes worn or not making good contact with slip rings	Check length of brushes and replace if worn excessively; Clean slip rings
AC Output Voltage Too Low Or Too High	If generator frequency is within specified limits but voltage is incorrect, transformer is incorrectly connected or defective	Adjust tap connections on transformer secondary windings to obtain correct voltage; Replace transformer if voltage cannot be corrected with adjustments
AC Output Voltage Too Low Or Too High	Open, grounded, or short circuit in rotor, stator, or transformer	Test each component for open, grounded, or shorted windings and replace if defective
Noisy Generator	Loose brush holder	Tighten brush holder
Noisy Generator	Worn generator end bearing	Replace end bearing
Noisy Generator	Rotor and stator rubbing together due to varnish lumps	Check for varnish lumps between rotor and stator and remove as required
Noisy Generator	Rotor and stator rubbing together due to rotor misaligned with crankshaft	Follow specified assembly procedures to correct rotor to crankshaft alignment
Generator Overheats	Generator overloaded due to defective circuit breaker CB22	Replace circuit breaker
Generator Overheats	Airflow restricted due to dirt or debris covering vent openings in stator housing	Clear away all dirt or debris as required
Generator Overheats	Stator windings covered with oil or dirt	Clean stator windings
Generator Overheats	Open, grounded, or short circuit in rotor, stator, or transformer	Test each component for open, grounded, or shorted windings and replace if defective
Generator Overheats	Incorrect transformer connection	Refer to Transformer Voltage Adjustments section

Fault codes and symptom→remedy steps extracted from this model’s service/owner documentation; verify against the official manual before servicing.

Cross-brand guides for these symptoms

Engine overheating Low oil pressure

Common questions

Cummins/Onan Spec C: Low oil pressure — what causes it?

Documented causes: Oil too light; Oil badly diluted; Oil too low; Oil relief valve not seating. See the table for the matching fixes.

Cummins/Onan Spec C: Engine backfires — what causes it?

Documented causes: Lean fuel mixture; Clogged fuel filter; Air leak at intake manifold or carburetor flange; Poor fuel. See the table for the matching fixes.

Cummins/Onan Spec C: Engine does not crank — what causes it?

Documented causes: Control fuse F11 open; Insufficient voltage for cranking due to terminal connection loose or corroded; Insufficient voltage for cranking due to battery dead or not charged; If engine cranks at set but not at the remote control panel, cause may be open circuit in remote control. See the table for the matching fixes.