Cummins/Onan 705JB troubleshooting: 20 fault codes and symptom-cause-remedy entries from the manufacturer's own service documentation.
Cummins/Onan 705JB — full specs, parts & maintenance.
All Cummins/Onan fault codes →
| Symptom | Cause | Remedy |
|---|---|---|
| Engine will not crank | Battery discharged, Loose connections, Defective starting circuit, Defective starting motor on revolving field generator, Defective switch | Recharge battery, Tighten connections, Repair or replace as necessary, Repair or replace as necessary, Replace |
| Engine cranks too stiffly | Too heavy oil in crankcase | Drain and refill with lighter oil |
| Engine will not start when cranked | Lack of fuel or faulty carburetion, Clogged fuel screen, Cylinders flooded, Poor fuel, Poor compression | Refill tank and check fuel system, Clean, adjust as necessary, Clean, Crank few times with spark plugs removed, Tighten spark plugs |
| Switch is released | None provided | Clean and adjust |
| Engine stops when started | Centrifugal switch remained open (units with revolving field generator) | Adjust centrifugal switch |
| Excessive oil consumption, smoky exhaust | Oil leaks from oil base or connections | Replace gaskets and tighten connections |
| Light blue smoke | Oil too light or diluted | Drain, refill with correct oil |
| Engine misfires | Faulty ignition, Engine misfiring, Too much oil | Clean, adjust or replace spark plugs, Clean, adjust or replace spark plugs, Drain excess oil |
| Engine races | Governor not controlling carburetor performance & linkage condition | Check governor performance and linkage condition |
| Black, smoky exhaust, excessive fuel consumption, fouling of spark plug with soot, possible lack of power under heavy load | Fuel mixture too rich, Choke not open, Dirty air cleaner, Excessive crankcase pressure, causing excessive fuel pump pressure | Adjust carburetor or choke and install needed parts, Inspect linkage and setting, Clean breather valve, Clean breather valve |
| Engine stops unexpectedly | Fuel tank empty | Fill with fresh fuel |
| Sharp metallic thud, especially when cold engine first started | Low oil supply, Oil badly diluted | Add oil, Change oil |
| Pinging sound when engine is suddenly or heavily loaded | Wrong spark plug, Spark plug burned or carboned, Fuel stale or low octane, Lean fuel mixture | Install correct spark plug, Install new plug, Use good, fresh fuel, Clean and adjust carburetor |
| Light pounding knock | Low oil supply, Oil badly diluted | Add oil, Change oil |
| Engine misfires at light load | Spark plug gap too narrow | Adjust to correct gap |
| Engine misfires at heavy load | Faulty ignition, Clogged carburetor | Clean, adjust or replace spark plugs, Clean jet and adjust carb |
| Generator backfires | Lean fuel mixture, Poor fuel | Clean or adjust carburetor, Refill with good, fresh fuel |
| High oil pressure | Defective gage, Oil too heavy grade, Clogged oil passages, Oil relief valve stuck | Replace, Drain and refill with proper oil, Clean all lines & passages, Clean by-pass |
| Engine overheating | Poor air circulation, Improper lubrication, Fuel mixture too lean | Maintain supply, See Low Oil Pressure, Adjust carburetor |
| Generator overheating (approximately 160°F higher than ambient) | Overloaded | Reduce load |
Fault codes and symptom→remedy steps extracted from this model’s service/owner documentation; verify against the official manual before servicing.
Documented causes: Battery discharged, Loose connections, Defective starting circuit, Defective starting motor on revolving field generator, Defective switch. See the table for the matching fixes.
Documented causes: Too heavy oil in crankcase. See the table for the matching fixes.
Documented causes: Lack of fuel or faulty carburetion, Clogged fuel screen, Cylinders flooded, Poor fuel, Poor compression. See the table for the matching fixes.