Cummins/Onan 15HQ-150R troubleshooting: 112 symptom-cause-remedy entries from the manufacturer's own service documentation.
Cummins/Onan 15HQ-150R — full specs, parts & maintenance.
All Cummins/Onan fault codes →
| Symptom | Cause | Remedy |
|---|---|---|
| Generator overheating | Overloaded | Reduce load |
| Generator overheating | Brush rig out of position | Be sure to line up marks |
| Voltage drops under heavy load | Engine lacks power | See remedies for engine missing under heavy load |
| Voltage drops under heavy load | Poor compression | Tighten cylinder head 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 replace parts necessary |
| Voltage drops under heavy load | Restricted air cleaner | Clean and refill |
| Voltage drops under heavy load | Excessive choking | See that choke opens properly |
| Voltage drops under heavy load | Carbon or lead in cylinder | Remove carbon |
| Voltage drops under heavy load | Restricted exhaust line | Clean or increase the size |
| Engine misfires at light load | Carburetor idle adjustment set wrong or clogged | Adjust, clean if needed |
| Engine misfires at light load | Spark plug gaps too narrow | Adjust to correct gap |
| Engine misfires at light load | Intake air leak | Tighten or replace gaskets |
| Engine misfires at light load | Faulty ignition | Clean, adjust, or replace breaker points, plugs, condenser, etc., or retime ignition |
| Engine misfires at light load | Uneven compression | Tighten cylinder head and spark plugs; If still not corrected, grind valves; Replace piston rings, if necessary |
| Engine misfires at light load | Worn intake valve stems or guides | Replace valves or guides |
| Engine misfires at heavy load | Spark plugs defective | Replace |
| Engine misfires at heavy load | Faulty ignition | Clean, adjust, or replace breaker points, plugs, condenser, etc., or retime ignition |
| Engine misfires at heavy load | Clogged carburetor | Clean jets |
| Engine misfires at heavy load | Clogged fuel screen | Clean |
| Engine misfires at heavy load | Defective spark plug cables | 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 | Sticking valves | Clean stems and guides |
| Engine misfires at all loads | Broken valve spring | Replace |
| Engine misfires at all loads | Defective ignition wires | Replace |
| Engine misfires at all loads | Defective or improperly adjusted 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 |
| Plant starts but does not continue to run | START button released too soon | Hold in contact longer |
| Plant starts but does not continue to run | Defective charging generator | Repair |
| Plant starts but does not continue to run | Defective panel equipment | See Controls |
| Engine backfires at carburetor | Lean fuel mixture | Clean carburetor |
| Engine backfires at carburetor | Clogged fuel screen | Clean screen |
| Engine backfires at carburetor | Intake air leak | Replace flange gaskets, tighten carb |
| Engine backfires at carburetor | Poor fuel | Refill with good, fresh fuel |
| Engine backfires at carburetor | Spark too late | Retime ignition |
| Engine backfires at carburetor | Spark plug wires crossed | Install wires correctly |
| Engine backfires at carburetor | Intake valves leaking | Grind or replace |
| Excessive oil consumption, light blue smoky exhaust | Worn piston rings | Install new piston rings |
| Excessive oil consumption, light blue smoky exhaust | Oil leaks from engine or connections. This does not cause smoky exhaust | Replace gaskets or leaking tubing. Tighten screws and connections |
| Excessive oil consumption, light blue smoky exhaust | Oil too light or diluted | Drain, refill with correct oil |
| Excessive oil consumption, light blue smoky exhaust | Too large bearing clearance | Replace bearings |
| Excessive oil consumption, light blue smoky exhaust | Oil pressure too high | Refer to symptoms of high oil pressure for remedies |
| Excessive oil consumption, light blue smoky exhaust | Engine misfires | Refer to symptoms of engine misfires |
| Excessive oil consumption, light blue smoky exhaust | Faulty ignition | Clean, adjust, or replace breaker points, plugs, condenser, etc., or retime ignition |
| Excessive oil consumption, light blue smoky exhaust | Unit operated at light or no load for long periods | No remedy needed |
| Excessive oil consumption, light blue smoky 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 | Adjust choke. Install needed carburetor parts, adjust float level |
| Black, smoky exhaust, excessive fuel consumption, fouling of spark plugs with black soot, possible lack of power under heavy load | Choke not 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, refill to proper level |
| Light pounding knock | Loose connecting rod bearing | Replace |
| Light pounding knock | Low oil supply | Add oil |
| Light pounding knock | Low oil pressure | Refer to symptom of low oil pressure for remedies |
| Light pounding knock | Oil badly diluted | Change oil |
| Engine stops unexpectedly | Fuel tank empty | Refill |
| Engine stops unexpectedly | Fuel pump failure | Repair or replace |
| Engine stops unexpectedly | High water temperature | See symptoms for engine overheating |
| Engine stops unexpectedly | Defective ignition | Check the ignition system. Repair or replace parts necessary |
| Dull metallic thud, if not bad, may disappear after few minutes operation, if bad, increases with load | Loose crankshaft | Replace bearings, unless one of the next three remedies permanently corrects the trouble |
| Sharp metallic thud, especially when cold engine first started | Low oil supply | Add oil |
| Sharp metallic thud, especially when cold engine first started | Low oil pressure | Refer to symptom of low pressure for remedies |
| Sharp metallic thud, especially when cold engine first started | Oil badly diluted | Change oil |
| Pinging sound when engine is rapidly accelerated or heavily loaded | Carbon in cylinders | Remove carbon |
| Pinging sound when engine is rapidly accelerated or heavily loaded | Spark too early | Retime ignition |
| Pinging sound when engine is rapidly accelerated or heavily loaded | Wrong spark plugs | Install correct plugs |
| Pinging sound when engine is rapidly accelerated or heavily loaded | Spark plugs burned or carboned | Install new plugs |
| Pinging sound when engine is rapidly accelerated or heavily loaded | Valves hot | Adjust tappet clearance |
| Pinging sound when engine is rapidly accelerated or heavily loaded | Fuel stale or low octane | Use good fresh fuel |
| Pinging sound when engine is rapidly accelerated or heavily loaded | Lean fuel mixture | Clean or adjust carburetor |
| Engine cranks too stiffly | Too heavy oil in crankcase | Drain, refill with lighter oil |
| Engine cranks too stiffly | Engine stuck | Disassemble and repair |
| Engine will not start when cranked | Faulty ignition | Clean, adjust, or replace breaker points, 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 | Clogged fuel screen | Clean |
| Engine will not start when cranked | Cylinders flooded | Crank few times 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 head and spark plugs. If still not corrected, grind the valves. Replace piston rings, if necessary |
| Engine will not start when cranked | Wrong timing | Retime ignition |
| Engine will not start when cranked | Poor choking | If plant is cold, adjust choke. If plant is warm, pull up on choke arm momentarily, while cranking |
| Engine runs but current does not build up | Poor brush contact or dirty commutator or slip rings | See that brushes seat well, are free in holders, are not worn too short, and have good spring tension |
| Engine runs but current does not build up | Open circuit, short circuit or ground in generator | See GENERATOR, replace part necessary |
| Current unsteady but engine not misfiring | Speed too low | Adjust governor to correct speed |
| Current unsteady but engine not misfiring | Poor commutation or brush contact | See that brushes seat well on commutator and slip rings, are free in holders, are not worn too short, and have good spring tension |
| Current unsteady but engine not misfiring | Loose connections | Tighten connections |
| Current unsteady but engine not misfiring | Fluctuating load | Correct any abnormal load condition |
| Current unsteady but engine not misfiring | Faulty voltage regulator | Adjust or replace, tighten connections |
| Tapping sound | Tappet clearance too great | Adjust or replace tappets |
| Tapping sound | Broken valve spring | Install new spring |
| Hollow clicking sound with cool engine under load | Loose pistons | If noise only slight and disappears when engine warms up, no immediate attention needed. Otherwise replace worn parts |
| Voltage low at far end of line but normal near power unit | Too small line wire 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 power unit | Too small line wire for load and distance | Install larger or extra wires, or reduce load |
| Noisy brushes | High mica between bars of commutator | Undercut mica |
| Excessive arcing of brushes | Rough commutator or rings | Turn down |
| Excessive arcing of brushes | Dirty commutator or rings | Clean |
| Excessive arcing of brushes | High mica | Undercut mica |
| Excessive arcing of brushes | Brush rig out of position | Line up marks on brush rig and support |
| Engine overheating | Low water in radiator | Refill radiator |
| Engine overheating | Overloaded | Remove part of load |
| Engine overheating | Improper lubrication | See Low Oil Pressure |
| Engine overheating | Radiator obstructed | Clean radiator |
| Engine overheating | Ignition timing late | Adjust ignition timing |
| Engine overheating | Improper ventilation | Provide for better air change |
Fault codes and symptom→remedy steps extracted from this model’s service/owner documentation; verify against the official manual before servicing.
Documented causes: Worn piston rings; Oil leaks from engine or connections. This does not cause smoky exhaust; Oil too light or diluted; Too large bearing clearance. See the table for the matching fixes.
Documented causes: Oil too light; Oil badly diluted; Oil too low; Oil relief valve not seating. See the table for the matching fixes.
Documented causes: Faulty ignition; Lack of fuel or faulty carburetion; Clogged fuel screen; Cylinders flooded. See the table for the matching fixes.