Cummins/Onan G855 Series troubleshooting: 18 fault codes and symptom-cause-remedy entries from the manufacturer's own service documentation.
Cummins/Onan G855 Series — full specs, parts & maintenance.
All Cummins/Onan fault codes →
| Symptom | Cause | Remedy |
|---|---|---|
| Coolant Loss - External | Radiator cap is not correct, is malfunctioning, or has low-pressure rating | Check the radiator pressure cap. Refer to the OEM service manual. |
| Coolant Temperature Above Normal - Gradual Overheat | Coolant level is below specification; Lubricating oil level is above or below specification; Load is excessive for engine horsepower rating; Coolant temperature gauge is malfunctioning; Radiator fins are damaged or obstructed with debris; Radiator cap is not correct, is malfunctioning, or has low-pressure rating; Cooling system hose is collapsed, restricted, or leaking | Inspect the engine and cooling system for external coolant leaks. Repair if necessary. Add coolant.; Beene enannne Check the oil level. Add or drain oil, if necessary.; Refer to Procedure 007-043 (Lubricating Oil Level) in Sections 3 and Procedure 007-002 (Lubricating Oil and Filters) in Section 4.; Reduce the load on the engine. Refer to OEM specifications.; Test the temperature gauge. Repair or replace the gauge if necessary. Refer to the OEM service manual.; Inspect the radiator fins. Clean and repair the fins as necessary. Refer to the OEM service manual.; Check the radiator pressure cap. Refer to the OEM service manual. |
| Coolant Temperature Below Normal | Engine is operating at low ambient temperature | OK |
| Engine Decelerates Slowly | Compare the calibration in the electronic control system with the engine rating and the Control Parts Electronic control module system calibration is |............ List (CPL), Bulletin number 3379133 or 4021327. If not correct necessary, calibrate the system. | Refer to a Cummins Authorized Repair Facility. |
| Throttle plate is malfunctioning | Refer to a Cummins Authorized Repair Facility. | |
| Engine Difficult to Start or Will Not Start | STEP 1 Check the OEM fuel shutoff valves. Verify that the OEM manual fuel shutoff valve is closed fuel tanks are open. | |
| Engine cranking speed is slower than 150 rpm | Refer to the Engine Will Not Crank or Cranks Slowly symptom tree. | |
| Air intake system restriction | STEP 3 Air intake system restriction is above specification | Replace the air filter. Refer to Procedure 010-059 (Air Cleaner Restriction) in Section 4 and Procedure 010-060 (Air Cleaner Element, Single Heavy Duty Dry Type) in Section 6. |
| Engine-driven units are engaged | Disengage engine-driven units. Verify the correct starting procedure. Refer to Procedure 101-014 (Normal Starting Procedure) in Section 1. | |
| Engine cranking speed is too slow | Inspect the air intake system for restriction. Replace the air filter. Refer to Procedure 010-059 (Air Cleaner Restriction) in Section 4 and Procedure 010-060 (Air Cleaner Element, Single Heavy Duty Dry Type) in Section 6. | |
| Engine noise excessive | STEP 1 Lubricating oil pressure is below specification STEP 2 Coolant temperature is above specification STEP 3 Fan drive belt is loose, tight, or not in alignment STEP 4 Engine mounts are worn, damaged, or not correct STEP 5 Ignition system is malfunctioning or misfiring STEP 6 Overhead adjustments are not correct | |
| Engine power output low | STEP 1 Verify the complaint STEP 2 Load is excessive for engine horsepower rating STEP 3 Engine is misfiring STEP 4 Air intake or exhaust leaks STEP 5 Air intake system restriction is above specification STEP 6 Low or no fuel pressure at the fuel filter (primary pressure) STEP 7 Low or no fuel pressure at the fuel filter (secondary pressure) | |
| Engine runs rough at idle | STEP 1 Engine mounts are worn, damaged, or not correct STEP 2 Ignition system is malfunctioning or misfiring STEP 3 Intake air restricted STEP 4 Overhead adjustmenis are not correct | |
| Engine runs rough or misfires | Fuel leak | |
| Engine Starts But Will Not Keep Running | Fuel valve closed (Manual), Load is excessive for engine horsepower rating, Air intake system restriction is above specification, Exhaust system resiriction is not within specification, Fuel supply pressure is below specification, Low or no fuel pressure at the mixer (secondary pressure) | Inspect the air intake system for restriction. Check the exhaust system for restrictions. Measure the fuel supply pressure. |
| Engine Vibration Excessive | Engine is misfiring, Fan is loose, damaged, or not balanced, Engine mounts are worn, damaged, or not correct | Check the fan. Check the engine mounts. |
| Engine Will Not Crank or Cranks Slowly (Air Starter) | Starting procedure is not correct, Crankshaft rotation is impaired, Starting air pressure low, Starting circuit component is malfunctioning | Verify the correct starting procedure. Check the crankshaft for ease of rotation. Check that the air supply valve is open completely. Check the air compressor for proper operation. Check the starting circuit components. |
| Engine Will Not Reach Rated Speed (RPM) | Load is excessive for engine horsepower rating | Reduce the load on the engine. |
Fault codes and symptom→remedy steps extracted from this model’s service/owner documentation; verify against the official manual before servicing.
Documented causes: Radiator cap is not correct, is malfunctioning, or has low-pressure rating. See the table for the matching fixes.
Documented causes: Coolant level is below specification; Lubricating oil level is above or below specification; Load is excessive for engine horsepower rating; Coolant temperature gauge is malfunctioning; Radiator fins are damaged or obstructed with debris; Radiator cap is not correct, is malfunctioning, or has low-pressure rating; Cooling system hose is collapsed, restricted, or leaking. See the table for the matching fixes.
Documented causes: Engine is operating at low ambient temperature. See the table for the matching fixes.