Cummins/Onan 6.5 NH troubleshooting: 7 fault codes and 140 symptom-cause-remedy entries from the manufacturer's own service documentation.
Cummins/Onan 6.5 NH — full specs, parts & maintenance.
All Cummins/Onan fault codes →
| Code | Meaning | Cause | Remedy |
|---|---|---|---|
| F1 | Fails to crank — Blown fuse (F1) | Blown fuse (F1) | Replace fuse (F1) on control. |
| K2 | Cranks but won't start — Faulty crank ignition relay (K2) | Faulty crank ignition relay (K2) | Check voltage from relay terminal 87 to ground while cranking unit. Battery voltage should appear at this terminal. If not, check for voltage at relay terminals 30 and 85. If battery voltage is present at terminals 30 and 85 but not at 87, place a ground lead on terminal 86 and recheck. Replace relay if voltage is not present with ground lead in place. If no voltage is present at terminals 30 and 85 while cranking, check wiring between start solenoid (K1) and crank ignition relay (K2). |
| R1 | Unit starts, but stops immediately after releasing start switch S1 — Resistor R1 may be open | Resistor R1 may be open | Check voltage on both sides of R1. With set running, voltage should be 24-32 volts DC. |
| K3 | Unit starts, but stops immediately after releasing start switch S1 — Run ignition relay K3 | Run ignition relay K3 | Check voltage on both sides of K3. Should be 12 volts. |
| S3 | Unit starts, but stops immediately after releasing start switch S1 — S3 low oil pressure switch may be defective | S3 low oil pressure switch may be defective | Check S3. Switch should close with set running at 10 lbs. minimum oil pressure. |
| F1 | Remote running time meter or generator lamp inoperative — Blown fuse (F1) | Blown fuse (F1) | Replace F1 fuse on control. |
| F1 | Control fuse (F1) is open | Replace fuse (5 ampere) |
| Symptom | Cause | Remedy |
|---|---|---|
| Fails to crank | Bad battery connection | Clean and tighten all battery and cable connections. |
| Fails to crank | Low battery | Check specific gravity. Recharge or replace if necessary. |
| Fails to crank | Faulty start solenoid (K1) | Check for battery voltage at K1 "I" terminal; push start switch, check start solenoid "S1" terminal voltage to ground; when battery voltage at start solenoid "B-" terminal is present, battery voltage should also appear at "S1" terminal; if not, check K3. |
| Fails to crank | Faulty start switch | Jumper switch (#3 terminal) to ground. If solenoid energizes, replace switch. |
| Cranks slowly | Bad battery connection | See 1 above (FAILS TO CRANK): Clean and tighten all battery and cable connections. |
| Cranks slowly | Low battery | See 2 above (FAILS TO CRANK): Check specific gravity. Recharge or replace if necessary. |
| Cranks but won't start | Faulty fuel solenoid or fuel pump | Fuel solenoid must open during cranking and running. Check by removing steel line from carburetor and crank engine. If fuel solenoid is open, fuel will pulsate out of this line. If it does not, the fuel solenoid and fuel pump must be checked separately to determine defective part. |
| Cranks but won't start | Faulty ignition | Check to see if points open and close during cranking. If they do not open and close, adjust and set points. Plug and plug wires must be in good condition. Voltage at ignition coil negative terminal (-) must alternate from +12 volts to zero volts as points open and close during engine cranking. |
| Cranks but won't start | Inoperative choke | With engine not running, check choke vane movement by pushing choke lever arm. Choke must be in closed position with cold engine and must be free to move against bimetal spring. As engine warms up, bimetal spring relaxes and allows choke vane to open fully. The lever will pulsate as engine warms up. See ADJUSTMENT section. |
| Unit starts, but stops immediately after releasing start switch S1 | Low oil level | Check oil level. If low or empty, refill to proper level. |
| Unit runs then stops | Low oil level | Check oil level. If low or empty, refill to proper level. |
| Unit runs but surges | Stuck choke | See 3 above (CRANKS BUT WON'T START): With engine not running, check choke vane movement by pushing choke lever arm. Choke must be in closed position with cold engine and must be free to move against bimetal spring. As engine warms up, bimetal spring relaxes and allows choke vane to open fully. The lever will pulsate as engine warms up. See ADJUSTMENT section. |
| Unit runs but surges | Governor not adjusted properly | Readjust governor. |
| Unit stops | Faulty ignition | See 4 above (CRANKS BUT WON'T START): Check to see if points open and close during cranking. If they do not open and close, adjust and set points. Plug and plug wires must be in good condition. Voltage at ignition coil negative terminal (-) must alternate from +12 volts to zero volts as points open and close during engine cranking. |
| Unit stops | Out of fuel | Refill fuel tank. |
| Unit stops | Low oil level | Check oil level. If low or empty, refill to proper level. |
| Engine misfires | Faulty ignition due to worn or fouled spark plugs | Clean or replace spark plugs |
| Engine misfires | Faulty ignition due to worn ignition points | Replace breaker points |
| Engine misfires | Faulty ignition due to incorrect ignition timing | Set breaker point gap |
| 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 (gasoline sets only) | 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 (gasoline sets only) | Drain fuel tank and refill with fresh fuel |
| Engine misfires | Carburetor icing (gasoline sets only) | In cold weather, place air preheater in winter position |
| Engine backfires | Faulty ignition due to incorrect ignition timing | Adjust breaker point gap |
| 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 (gasoline sets only) | 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 Engine Block Assembly |
| Engine lacks power | Faulty ignition due to incorrect ignition timing | Adjust breaker point gap |
| Engine lacks power | Faulty ignition due to incorrect spark plug gap | Reset spark plug gap |
| Engine lacks power | Dirty air cleaner | Replace air cleaner |
| Engine lacks power | Restricted fuel flow due to plugged fuel filter | Clean fuel filter |
| Engine lacks power | Restricted fuel flow due to faulty fuel pump | Test fuel pump and repair or 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 (gasoline sets only) | 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 tappet clearance | Adjust valve tappets (see Engine Block Assembly section) |
| Engine lacks power | Excessive engine wear or damage to engine | See Engine Block Assembly section |
| Engine lacks power | Carburetor air preheater set incorrectly | In hot weather, place air preheater in summer position |
| Engine black exhaust | 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 black exhaust | Dirt or oil covering engine cooling fins | Clean away all dirt and oil from engine cooling fins |
| Engine black exhaust | Incorrect ignition timing | Adjust breaker point gap |
| Engine black exhaust | Lean fuel mixture due to incorrectly adjusted fuel mixture screws | Adjust carburetor main and idle adjustment screws |
| Engine black exhaust | Lean fuel mixture due to incorrect float level (gasoline sets only) | Adjust carburetor float level |
| Engine black exhaust | Lean fuel mixture due to dirt in carburetor | Disassemble carburetor and clean all internal passages |
| Engine black exhaust | Rich fuel mixture due to dirty air cleaner | Replace air cleaner |
| Engine black exhaust | Rich fuel mixture due to choke sticking (gasoline sets only) | Clean choke and choke linkage |
| Engine black exhaust | Rich fuel mixture due to incorrectly adjusted fuel mixture screws | Adjust carburetor idle and main adjustment screws |
| Engine black exhaust | Rich fuel mixture due to dirt in carburetor | Disassemble carburetor and clean all internal passages |
| Engine hunts or surges | Sticking or binding governor linkage | Clean and lubricate governor linkage |
| Engine hunts or surges | Incorrect governor adjustment | Adjust governor speed and sensitivity |
| Engine hunts or surges | Faulty governor spring | 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 (gasoline sets only) | Adjust carburetor float level |
| Engine hunts or surges | Incorrect fuel mixture due to dirt in carburetor | Disassemble carburetor and clean all internal passages |
| Engine hunts or surges | Governor mechanism worn excessively | See Engine Block Assembly section |
| Engine hunts or surges | Carburetor icing (gasoline sets only) | In cold weather, place air preheater in winter position |
| 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 Engine Block Assembly section |
| 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 Engine Block Assembly section) |
| Low oil pressure | Faulty oil bypass valve | Inspect oil bypass valve, clean or replace as required (see Engine Block Assembly section) |
| Low oil pressure | Excessive engine wear or defective oil pump | See Engine Block Assembly section |
| Engine does not crank | If engine cranks at set but not at remote control panel, fault is due to 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 remote control panel, fault is due to remote start switch faulty | Replace remote start control switch |
| Engine does not crank | If engine cranks at remote control panel but not at set, fault is due to open circuit in wiring between S1 switch and PC board | Check for continuity and correct if circuit is open |
| Engine does not crank | If engine cranks at remote control panel but not at set, fault is due to faulty S1 switch | Replace S1 switch |
| Engine does not crank | Insufficient voltage for cranking due to battery not charged | Check condition of battery and recharge or replace |
| Engine does not crank | Insufficient voltage for cranking due to terminal connections loose or dirty | Clean and tighten all connections at battery, K1 start solenoid, and starter motor |
| Engine does not crank | With battery voltage present at terminal S on start solenoid when S1 is placed in Start position, fault is due to K1 relay not grounded | Tighten solenoid bracket mounting screw |
| Engine does not crank | If voltage is not present at terminal S on start solenoid when S1 is placed in Start position, fault is due to open circuit between K1 relay and control PC board | Check for continuity and correct if circuit is open |
| Engine cranks but does not start | Faulty ignition due to worn or fouled spark plugs, worn ignition points, incorrect ignition timing, or faulty plug wires | 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, or carburetor mixture screws incorrectly adjusted | 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 when S1 is placed in Start position, fault is due to open circuit between coil and control | Check for continuity and correct if circuit is open |
| Engine starts but stops when start switch is released | Low oil pressure switch not closing due to low oil level | Check oil level and add oil if low |
| Engine starts but stops when start switch is released | Low oil pressure switch not closing due to open circuit between switch and control | Check for continuity and correct if circuit is open |
| Engine starts but stops when start switch is released | Low oil pressure switch not closing due to defective low oil pressure switch | Replace low oil pressure switch |
| Engine starts but stops when start switch is released | Low oil pressure switch not closing due to low oil pressure | Refer to Lubrication system section for test and service procedures |
| Engine starts but stops when start switch is released | Output voltage from generator not being supplied to control due to defective charge resistor | Test R6 charge resistor (2.5 ohm) and replace if defective |
| Engine starts but stops when start switch is released | Output voltage from generator not being supplied to control due to open circuit in wiring between generator and control | Check for continuity and correct if circuit is open |
| Engine starts but stops when start switch is released | Output voltage from generator not being supplied to control due to defective bridge rectifier | Refer to Generator section for test and service procedures |
| Engine starts but stops when start switch is released | Output voltage from generator not being supplied to control due to no output voltage from generator | Refer to Generator section for test and service procedures |
| Engine starts but stops when start switch is released | Defective control PC board | Replace control PC board |
| Engine starts and runs; then stops. Set restarts immediately or set restarts after cooling down | Fuel level is below generator set fuel pickup tube or oil level is low | Check fuel and oil levels and refill as necessary |
| Engine starts and runs; then stops. Set restarts immediately or set restarts after cooling down | Faulty choke operation due to sticking choke linkage, incorrect choke adjustment, open circuit in wiring between choke heater and generator, or defective choke heater | Refer to Fuel System section for testing and service procedures |
| Engine starts and runs; then stops. Set restarts immediately or set restarts after cooling down | Vapor lock due to high ambient air temperature | Remove any objects or debris that may restrict airflow to and from the set |
| Engine starts and runs; then stops. Set restarts immediately or set restarts after cooling down | Vapor lock due to faulty fuel pump | Refer to Fuel System section for test and service procedures |
| Engine starts and runs; then stops. Set restarts immediately or set restarts after cooling down | Breaker points sticking | Replace breaker points |
| Engine starts and runs; then stops. Set restarts immediately or set restarts after cooling down | Contaminated fuel | Refill tank with fresh fuel |
| Low battery | Weak or discharged battery; generator set charger will not recharge a battery that is in a very low state of charge | Connect a separate battery charger to bring battery up to full charge |
| Low battery | Load connected to battery while set is turned off | Turn off load |
| Low battery | If ammeter shows battery is discharging, fault is due to defective generator charge winding | Refer to Generator section for testing and service procedures |
| Low battery | If ammeter shows battery is discharging, fault is due to CR1 power rectifier open | Replace control PC board |
| Run lamp, time meter, or battery condition meter does not operate | Open circuit between terminal 6 or remote connector plug and terminal 6 on Start/Run/Stop switch | Check for continuity and correct if circuit is open |
| Run lamp, time meter, or battery condition meter does not operate | Open circuit between ground terminals on lamp or meters and terminal 1 on remote Start/Run/Stop switch | Check for continuity and correct if circuit is open |
| Run lamp, time meter, or battery condition meter does not operate | If battery condition meter works but time meter does not operate, time meter is defective | Replace time meter |
| Run lamp, time meter, or battery condition meter does not operate | If time meter works but battery condition meter does not operate, verify that jumper wire is connected between terminals 5 and 6 on remote start/stop switch; if jumper is in place and reading equals battery voltage minus 10 volts, battery meter is defective | Replace battery condition meter |
| Run lamp, time meter, or battery condition meter does not operate | If reading does not equal battery voltage minus 10 volts, zener diode is defective | Replace zener diode |
| Engine Does Not Crank | If engine cranks at set but not at remote control panel, fault is due to: open circuit in remote control wiring | Check for continuity and correct if circuit is open |
| Engine Does Not Crank | Connect a voltmeter between terminal S on the start solenoid and ground. Check for battery voltage when S1 is placed in Start position. If voltage is present, fault is due to: K1 relay not grounded | Tighten solenoid bracket mounting screws |
| Engine Does Not Crank | If voltage is not present as described in step 5 test, fault is due to: open circuit between K1 relay and control PC board | Check for continuity and correct if circuit is open |
| Engine Does Not Crank | If voltage is not present as described in step 5 test, fault is due to: defective control PC board | Replace control PC board |
| Engine Cranks But Does Not Start | Faulty fuel system due to carburetor mixture screws incorrectly adjusted, plugged fuel filter, faulty regulator, or faulty fuel solenoid | Refer to Fuel System section for test and service procedures |
| Engine Cranks But Does Not Start | Connect a voltmeter between positive (+) terminal on coil and ground. Check for battery voltage when S1 is placed in the Start position. If voltage is not present, fault is due to: open circuit between coil and control | Check for continuity and correct if circuit is open |
| Engine Starts But Stops When Start Switch Is Released | K5 relay contacts not opening due to: K5 relay circuit open | Check for continuity and correct if circuit is open |
| Engine Starts But Stops When Start Switch Is Released | K5 relay contacts not opening due to: defective K5 relay | Replace K5 relay |
| Engine Starts But Stops When Start Switch Is Released | Output voltage from generator not being supplied to control due to: defective bridge rectifier, or no output voltage from generator | Refer to Generator section for test and service procedures |
| Engine Starts And Runs; Then Stops. Set Restarts Immediately or Set Restarts After Waiting Several Minutes | Oil level is low | Check fuel and oil levels and refill as necessary |
| Engine Starts And Runs; Then Stops. Set Restarts Immediately or Set Restarts After Waiting Several Minutes | Dirty fuel restricting fuel filter causing filter to freeze up | Clean fuel filter. Refer to Fuel System section for service procedures |
| Engine Starts And Runs; Then Stops. Set Restarts Immediately or Set Restarts After Waiting Several Minutes | Breaker points sticking | Replace breaker points |
| Engine Starts And Runs; Then Stops. Set Restarts Immediately or Set Restarts After Waiting Several Minutes | Contaminated fuel | Refill tank with fresh fuel |
| Low Battery | Disconnect the negative (-) battery cable at the battery and remove control lead P1-6 from the battery positive (+) terminal on the K1 start solenoid. Connect a DC ammeter and 5 ampere fuse in series between the P1-6 lead wire and the stud terminal on the K1 solenoid. With set running, the ammeter should read between 0 and 1 ampere. If meter shows battery is discharging, fault is due to: defective generator charge winding | Refer to Generator section for testing and service procedures |
| 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 CR4 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 CR4 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 CR4 bridge rectifier (causes low AC output voltage) | Test CR4 rectifier and replace if defective |
| AC Output Voltage Too Low Or Too High | Brushes worn or not making good contact with slip rings (causes low AC output voltage) | Check length of brushes and replace if worn excessively. Clean or replace 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 (causes low AC output voltage) | 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 | Remove part of load and 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 |
Fault codes and symptom→remedy steps extracted from this model’s service/owner documentation; verify against the official manual before servicing.
Documented causes: Faulty ignition due to incorrect ignition timing; Faulty ignition due to incorrect spark plug gap; Dirty air cleaner; Restricted fuel flow due to plugged fuel filter. See the table for the matching fixes.
Documented causes: Faulty ignition due to worn or fouled spark plugs; Faulty ignition due to worn ignition points; Faulty ignition due to incorrect ignition timing; Faulty ignition due to faulty ignition coil. See the table for the matching fixes.
Documented causes: Restricted airflow due to dirt or debris blocking air inlet or outlet; Dirt or oil covering engine cooling fins; Incorrect ignition timing; Lean fuel mixture due to incorrectly adjusted fuel mixture screws. See the table for the matching fixes.