Cummins/Onan 15HQ-150R troubleshooting: 24 fault codes and 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 |
|---|---|---|
| Engine lacks power. | Tighten cylinder head and spark plugs.<br>If still not corrected, grind the valves,<br>Replace piston rings, if necessary. | Check the fuel system. Clean, adjust,<br>or replace parts necessary. |
| Engine misfires at light load. | Carburetor idle adjustment Adjust, clean if needed,<br>set wrong or clogged.<br>Spark plug gaps too narrow. Adjust to correct gap.<br>Intake air leak.<br>Faulty ignition.<br>Tighten or replace gaskets,<br>Clean, adjust, or replace breaker points, plugs, condenser, etc., or retime ignition.<br>Uneven compression. Tighten cylinder head and spark plugs.<br>If still not corrected, grind valves,<br>Replace piston rings, if necessary. | |
| Engine misfires at heavy load. | Spark plugs defective. Replace.<br>Clean, adjust, or replace breaker points, plugs, condenser, etc., or retime ignition.<br>Clogged carburetor. Clean jets.<br>Clogged fuel screen. Clean.<br>Replace.<br>Faulty ignition.<br>Defective spark plug cables. | |
| Engine misfires at all loads. | Fouled spark plug. Clean and adjust.<br>Wrong or effective spark plug. Replace.<br>Ticking valves. Clean stems and guides.<br>Broken valve spring. Replace.<br>Defective ignition wires. Replace.<br>Effective or improperly adjusted Adjust or replace breaker points. | |
| Low oil pressure. | Oil too light. Drain, refill with proper oil.<br>Dil badly diluted. Drain, refill with proper oil.<br>Oil too low. Add oil.<br>Dil relief valve not seating. Remove and clean, or replace.<br>Badly worn bearings. Replace.<br>Sludge on oil screen. Remove and clean.<br>Badly worn oil pump. Replace.<br>Defective oil pressure gauge. Replace. | |
| High oil pressure. | Oil too heavy. Drain, refill with proper oil.<br>Slogged oil passage. Clean all lines and passages.<br>Oil relief valve stuck. Remove and clean.<br>Defective oil pressure gauge. Replace. | |
| Plant starts but does not continue to run. | Start button released too soon. Hold in contact longer.<br>Defective charging generator. Repair.<br>Defective panel equipment. See Controls. | |
| Engine backfires at carburetor. | Lean fuel mixture. Clean carburetor.<br>Slogged fuel screen. Clean screen.<br>Intake air leak. Replace flange gaskets, tighten carb.<br>Poor fuel. Refill with good, fresh fuel. | |
| Excessive oil consumption, light blue smoky exhaust. | Worn piston rings. Install new piston rings.<br>Oil leaks from engine or con-nections. This does not cause smoky exhaust.<br>Oil too light or diluted. Drain, refill with correct oil,<br>Too large bearing clearance. Replace bearings.<br>Oil pressure too high. Refer to symptoms of high oil pressure for remedies. | |
| 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.<br>Choke not open. See that choke opens properly.<br>Dirty air cleaner. Clean, refill to proper level. | |
| Light pounding knock. | Loose connecting rod bearing. Replace,<br>Low oil supply. Add oil,<br>Low oil pressure. Refer to symptom of low oil pressure for remedies.<br>Oil badly diluted. Change oil. | |
| Engine stops unexpectedly. | Fuel tank empty. Refill.<br>Fuel pump failure. Repair or replace.<br>High water temperature. See symptoms for engine overheating.<br>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.<br>Sharp metallic thud, especially when cold engine first started. Add oil,<br>Refer to symptom of low pressure for remedies.<br>Oil badly diluted. Change oil. | |
| Pinging sound when engine is rapidly accelerated or heavily loaded. | Low oil supply.<br>Low oil pressure.<br>Carbon in cylinders. Remove carbon.<br>Spark too early. Retime ignition.<br>Wrong spark plugs. Install correct plugs.<br>Spark plugs burned or carboned. Install new plugs.<br>Valves hot. Adjust tappet clearance<br>Fuel stale or low octane. Use good fresh fuel.<br>Lean fuel mixture. | |
| Jil too heavy. | Drain, refill with proper oil. | |
| 'logged oil passage. | Clean all lines and passages. | |
| Jil relief valve stuck. | Remove and clean. | |
| Plant starts but does not continue to run | START button released too soon. | Hold in contact longer |
| Engine backfires at carburetor | Lean fuel mixture. | Clean carburetor. |
| Excessive oil consumption, light blue smoky exhaust | Worn piston rings. | Install new piston rings. |
| Engine misfires | Faulty ignition. | Clean, adjust, or replace breaker points, plugs, condenser, etc., or retimes ignition. |
| Unit operated at light or no load for long periods | No remedy needed. | |
| Too much oil | Drain excess oil. | |
| Light pounding knock | Loose connecting rod bearing. | Replace. |
Fault codes and symptom→remedy steps extracted from this model’s service/owner documentation; verify against the official manual before servicing.
Documented causes: Tighten cylinder head and spark plugs.<br>If still not corrected, grind the valves,<br>Replace piston rings, if necessary.. See the table for the matching fixes.
Documented causes: START button released too soon.. See the table for the matching fixes.
Documented causes: Lean fuel mixture.. See the table for the matching fixes.