Cummins/Onan G855 Series troubleshooting: 103 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 | Coolant level is above specification | Check the coolant level. Refer to Procedure 008-066 (Coolant Level) in Section 3. |
| Coolant Loss - External | External coolant leak | Inspect the engine for coolant leaking from hoses, draincocks, water manifold, expansion and pipe plugs, fittings, radiator core, exhaust heat shield, heat exchanger, air compressor and cylinder head gaskets, lubricating oil cooler, water pump seal, and OEM-mounted components that have coolant flow. |
| 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 Loss - External | Fill line or vent lines are restricted, obstructed, or not routed correctly | Check the vent lines and the fill line for correct routing and for restriction. Refer to OEM specifications. |
| Coolant Loss - External | Coolant fill line is restricted or obstructed | Check the coolant fill line for restrictions or obstructions. Refer to the OEM installation instructions. |
| Coolant Loss - External | Engine is overheating | Refer to Coolant Temperature Above Normal symptom tree. |
| Coolant Temperature Above Normal - Gradual Overheat | Coolant level is below specification | Inspect the engine and cooling system for external coolant leaks. Repair if necessary. Add coolant. Refer to Procedure 018-018 (Cooling System) in Section V. |
| Coolant Temperature Above Normal - Gradual Overheat | Lubricating oil level is above or below specification | 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. |
| Coolant Temperature Above Normal - Gradual Overheat | Load is excessive for engine horsepower rating | Reduce the load on the engine. Refer to OEM specifications. |
| Coolant Temperature Above Normal - Gradual Overheat | Coolant temperature gauge is malfunctioning | Test the temperature gauge. Repair or replace the gauge if necessary. Refer to the OEM service manual. |
| Coolant Temperature Above Normal - Gradual Overheat | Radiator fins are damaged or obstructed with debris | Inspect the radiator fins. Clean and repair the fins as necessary. Refer to the OEM service manual. |
| Coolant Temperature Above Normal - Gradual Overheat | 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 | Cooling system hose is collapsed, restricted, or leaking | Inspect the hoses. Refer to OEM installation instructions. |
| Coolant Temperature Below Normal | Engine is operating at low ambient temperature | Check the shutters and engine compartment air. Refer to Cold Weather Operation, Bulletin 3377710. |
| Coolant Temperature Below Normal | Temperature gauge malfunction | Check or replace the temperature gauge. Refer to the OEM service manual. |
| Coolant Temperature Below Normal | Thermostat is not operating properly | Check the thermostat for correct operation. Refer to a Cummins Authorized Repair Facility. |
| Engine Acceleration or Response Poor | Verify the complaint | Refer to the Human Machine Interface, if available or an electronic service tool to verify system faults and system status. Contact a Cummins Authorized Repair Facility if an electronic service tool is required. |
| Engine Acceleration or Response Poor | Load is excessive for engine horsepower rating | Reduce the load on the engine. Refer to the OEM service manual. |
| Engine Acceleration or Response Poor | Air intake or exhaust leaks | Check for loose or damaged piping connections and missing pipe plugs. Check the turbocharger and exhaust manifold mounting. Refer to the OEM service manual. |
| Engine Acceleration or Response Poor | Air intake system restriction is above specification | 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 Acceleration or Response Poor | Exhaust system restriction is not within specification | Check the exhaust system for restrictions. Contact a Cummins Authorized Repair Facility. |
| Engine Acceleration or Response Poor | Ignition system is malfunctioning or misfiring | Check the ignition system. Refer to Procedure 013-016 (Spark Plugs) in Section 5 and Procedure 013-014 (Spark Plug Wire) in Section 6. If the problem persists contact a Cummins Authorized Repair Facility. |
| Engine Decelerates Slowly | Electronic control module system calibration is not correct | Compare the calibration in the electronic control system with the engine rating and the Control Parts List (CPL), Bulletin number 3379133 or 4021327. If necessary, calibrate the system. Refer to a Cummins Authorized Repair Facility. |
| Engine Decelerates Slowly | Throttle plate is malfunctioning | Check the throttle plate for sticking or hysteresis. Refer to a Cummins Authorized Repair Facility. |
| Engine Difficult to Start or Will Not Start | OEM manual fuel shutoff valve is closed | Check the OEM fuel shutoff valves. Verify that the fuel tanks are open. |
| Engine Difficult to Start or Will Not Start | Engine cranking speed is too slow | If the cranking speed is slower than 150 rpm, refer to the Engine Will Not Crank or Cranks Slowly symptom tree. |
| Engine Difficult to Start or Will Not Start | Air intake system restriction is above specification | 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 Difficult to Start or Will Not Start | Engine-driven units are engaged | Disengage engine-driven units. |
| Engine Difficult to Start or Will Not Start | Starting procedure is not correct | Verify the correct starting procedure. Refer to Procedure 101-014 (Normal Starting Procedure) in Section 1. |
| Engine Noise Excessive | Lubricating oil pressure is below specification | Check the oil pressure. If the pressure is low, refer to the Lubricating Oil Pressure Low symptom tree. |
| Engine Noise Excessive | Coolant temperature is above specification | Refer to the Coolant Temperature is Above Normal symptom tree. |
| Engine Noise Excessive | Fan drive belt is loose, tight, or not in alignment | Check the fan drive belt. Refer to Procedure 008-087 (Cooling Fan Belt Tensioner) in Section 5 and Procedure 008-002 (Drive Belts) in Section 7. |
| Engine Noise Excessive | Engine mounts are worn, damaged, or not correct | Check the engine mounts. Refer to the OEM service manual. |
| Engine Noise Excessive | Ignition system is malfunctioning or misfiring | Check the ignition system. Refer to Procedure 013-016 (Spark Plugs) in Section 5 and Procedure 013-014 (Spark Plug Wire) in Section 6. If the problem persists contact a Cummins Authorized Repair Facility. |
| Engine Noise Excessive | Overhead adjustments are not correct | Measure and adjust the overhead settings. Refer to Procedure 003-004 (Overhead Set) Section 4. |
| Engine Power Output Low | Verify the complaint | Test the engine operation while under load. Perform an engine acceleration test. Perform an engine load/install test. Refer to the OEM installation instructions. |
| Engine Power Output Low | Load is excessive for engine horsepower rating | Reduce the load on the engine. Refer to OEM specifications. |
| Engine Power Output Low | Engine is misfiring | Refer to the Engine Runs Rough or Misfires symptom tree. |
| Engine Power Output Low | Air intake or exhaust leaks | Check for loose or damaged piping connections and missing pipe plugs. Check the turbocharger and exhaust manifold mounting. Refer to the OEM service manual. |
| Engine Power Output Low | Air intake system restriction is above specification | Check the air intake system for restriction. Clean or replace the air filter and inlet piping as necessary. 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 Power Output Low | Low or no fuel pressure at the fuel filter (primary pressure) | Increase output pressure from the primary regulator. Refer to a Cummins Authorized Repair Facility. |
| Engine Power Output Low | Low or no fuel pressure at the fuel filter (secondary pressure) | Increase output pressure from the secondary regulator. Refer to a Cummins Authorized Repair Facility. |
| Engine Runs Rough at Idle | Engine mounts are worn, damaged, or not correct | Inspect the engine mounts. Replace as needed. Refer to a Cummins Authorized Repair Facility. |
| Engine Runs Rough at Idle | Ignition system is malfunctioning or misfiring | Check the ignition system. Refer to Procedure 013-016 (Spark Plugs) in Section 5 and Procedure 013-014 (Spark Plug Wire) in Section 6. If the problem persists contact a Cummins Authorized Repair Facility. |
| Engine Runs Rough at Idle | Intake air restricted | Inspect and change air cleaner. Look for other restrictions. 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 Runs Rough at Idle | Overhead adjustments are not correct | Measure and adjust the overhead settings. Refer to Procedure 003-004 (Overhead Set) Section 4. |
| Engine Runs Rough or Misfires | Ignition system is malfunctioning or misfiring | Check the ignition system. Refer to Procedure 013-016 (Spark Plugs) in Section 5 and Procedure 013-014 (Spark Plug Wire) in Section 6. If the problem persists contact a Cummins Authorized Repair Facility. |
| Engine Speed Surges at Low or High Idle | Engine is cold | Allow the engine to warm up for approximately 1 to 3 minutes. Refer to Procedure 101-999 (Operating Instructions) in Section 1. |
| Engine Speed Surges at Low or High Idle | Ignition system is malfunctioning or misfiring | Check the ignition system. Refer to Procedure 013-016 (Spark Plugs) in Section 5 and Procedure 013-014 (Spark Plug Wire) in Section 6. If the problem persists contact a Cummins Authorized Repair Facility. |
| Engine Starts But Will Not Keep Running | Manual fuel valve closed | Open manual fuel valve. |
| Engine Starts But Will Not Keep Running | Load is excessive for engine horsepower rating | Reduce the load on the engine. Refer to OEM specifications. |
| Engine Starts But Will Not Keep Running | Air intake system restriction is above specification | 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 Starts But Will Not Keep Running | Exhaust system restriction is not within specification | Check the exhaust system for restrictions. Contact a Cummins Authorized Repair Facility. |
| Engine Starts But Will Not Keep Running | Fuel supply pressure is below specification | Measure the fuel supply pressure. Refer to a Cummins Authorized Repair Facility. |
| Engine Starts But Will Not Keep Running | Low or no fuel pressure at the mixer (secondary pressure) | Increase output pressure from the secondary regulator. |
| Engine Transient Response Poor | Load is excessive for engine horsepower rating | Reduce the load on the engine. Refer to OEM specifications. |
| Engine Transient Response Poor | Air intake or exhaust leaks | Inspect the air intake and exhaust systems for air leaks. Refer to the OEM service manual. |
| Engine Transient Response Poor | Air intake system restriction is above specification | 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 Transient Response Poor | Exhaust system restriction is not within specification | Check the exhaust system for restrictions. Contact a Cummins Authorized Repair Facility. |
| Engine Transient Response Poor | Ignition system is malfunctioning or misfiring | Check the ignition system. Refer to Procedure 013-016 (Spark Plugs) in Section 5 and Procedure 013-014 (Spark Plug Wire) in Section 6. If the problem persists contact a Cummins Authorized Repair Facility. |
| Engine Transient Response Poor | Low or no fuel pressure at the fuel filter (primary pressure) | Increase output pressure from the primary regulator. Refer to a Cummins Authorized Repair Facility. |
| Engine Transient Response Poor | Low or no fuel pressure at the fuel filter (secondary pressure) | Increase output pressure from the secondary regulator. Refer to a Cummins Authorized Repair Facility. |
| Engine Vibration Excessive | Engine is misfiring | Refer to the Engine Runs Rough or Misfires symptom tree. |
| Engine Vibration Excessive | Fan is loose, damaged, or not balanced | Check the fan. Refer to Procedure 008-040 (Fan, Cooling) in Section 3. |
| Engine Vibration Excessive | Engine mounts are worn, damaged, or not correct | Check the engine mounts. Refer to Procedure 016-010 (Engine Mounts) in Section 6. |
| Engine Will Not Crank or Cranks Slowly (Air Starter) | Starting procedure is not correct | Verify the correct starting procedure. Refer to Procedure 101-014 (Cold Weather Starting) in Section 1. |
| Engine Will Not Crank or Cranks Slowly (Air Starter) | Crankshaft rotation is impaired | Check the crankshaft for ease of rotation. Refer to Procedure 101-018 (Starting Procedure After Extended Shutdown or Oil Change) in Section 1. |
| Engine Will Not Crank or Cranks Slowly (Air Starter) | Starting air pressure low | Check that the air supply valve is open completely. Check the air compressor for proper operation. |
| Engine Will Not Crank or Cranks Slowly (Air Starter) | Starting circuit component is malfunctioning | Check the starting circuit components. Refer to the OEM service manual. |
| Engine Will Not Reach Rated Speed (RPM) | Load is excessive for engine horsepower rating | Reduce the load on the engine. Refer to OEM specifications. |
| Engine Will Not Reach Rated Speed (RPM) | Tachometer is not calibrated or is malfunctioning | Compare the tachometer reading with a handheld tachometer or an electronic service tool reading. Calibrate or replace the tachometer as necessary. Refer to the OEM service manual. |
| Engine Will Not Reach Rated Speed (RPM) | Air intake or exhaust leaks | Inspect the air intake and exhaust systems for air leaks. Refer to the OEM Service Manual. |
| Engine Will Not Reach Rated Speed (RPM) | Air intake system restriction is above specification | 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 Will Not Reach Rated Speed (RPM) | Engine power output is low | Refer to the Engine Power Output Low symptom tree. |
| Engine Will Not Reach Rated Speed (RPM) | Ignition system is malfunctioning or misfiring | Check the ignition system. Refer to Procedure 013-016 (Spark Plugs) in Section 5 and Procedure 013-014 (Spark Plug Wire) in Section 6. If the problem persists contact a Cummins Authorized Repair Facility. |
| Engine Will Not Reach Rated Speed (RPM) | Fuel supply pressure is below specification | Measure the fuel supply pressure. Refer to a Cummins Authorized Repair Facility. |
| Engine Will Not Reach Rated Speed (RPM) | Low or no fuel pressure at the mixer (secondary pressure) | Increase output pressure from the secondary regulator. |
| Engine Will Not Shut Off | Stop/Run switch circuit is malfunctioning | Check the Stop/Run switch circuit. Refer to the OEM service manual. |
| Engine Will Not Shut Off | Engine is running on fumes drawn into the air intake | Check the air intake ducts. Locate and isolate the source of the fumes. Repair as necessary. Refer to the OEM installation and service manual. |
| Fuel Consumption Excessive | Verify the complaint | Refer to the engine datasheet. Compare the fuel usage records with the engine datasheet. |
| Fuel Consumption Excessive | Load is excessive for engine horsepower rating | Reduce the load on the engine. Refer to OEM specifications. |
| Fuel Consumption Excessive | Fuel consumption has increased after an engine repair | Evaluate the engine repair to determine its effect on fuel consumption. Check part numbers to make sure the correct parts were used. Refer to the Control Parts List (CPL), Bulletin Number 4021326 or 4021326. |
| Fuel Consumption Excessive | Exhaust system restriction is not within specification | Check the exhaust system for restrictions. Contact a Cummins Authorized Repair Facility. |
| Fuel Consumption Excessive | Air intake system restriction is above specification | Check the air intake system for restriction. Clean or replace the air filter and inlet piping as necessary. 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. |
| Fuel Consumption Excessive | Ignition system is malfunctioning or misfiring | Check the ignition system. Refer to Procedure 013-016 (Spark Plugs) in Section 5 and Procedure 013-014 (Spark Plug Wire) in Section 6. If the problem persists contact a Cummins Authorized Repair Facility. |
| Lubricating Oil Consumption Excessive | Lubricating oil leak (external) | Inspect the engine for external oil leaks. Tighten the capscrews, pipe plugs, and fittings. Replace gaskets, if necessary. Refer to Procedure 018-009 (Capscrew Markings and Torque Values) in Section V torque for specifications. |
| Lubricating Oil Consumption Excessive | Lubricating oil level is above or below specification | Check the oil level. Add or drain oil, if necessary. Refer to Procedure 007-043 (Lubricating Oil Level) in Section 3 and Procedure 007-002 (Lubricating Oil and Filters) in Section 4. |
| Lubricating Oil Consumption Excessive | Engine is cold | Allow the engine to warm to operating temperature. If the engine will not reach operating temperature, refer to the Coolant Temperature Below Normal symptom tree. |
| Lubricating Oil Consumption Excessive | Lubricating oil does not meet specifications for operating conditions | Change the oil and filters. Refer to Procedure 007-002 (Lubricating Oil and Filters) in Section 4. Use the oil recommended in Section V. |
| Lubricating Oil Pressure High | Lubricating oil pressure switch, gauge, or sensor is malfunctioning or is not in the correct location | Check the oil pressure switch, gauge, or sensor for correct operation and location. Refer to the OEM service manual. |
| Lubricating Oil Pressure High | Coolant temperature is below specification | Refer to the Coolant Temperature Below Normal symptom tree. |
| Lubricating Oil Pressure High | Lubricating oil does not meet specifications for operating conditions | Change the oil and filters. Refer to Procedure 007-002 (Lubricating Oil and Filters) in Section 4. Use the oil type recommended in Section V. |
| Lubricating Oil Pressure Low | Lubricating oil level is above or below specification | Check the oil level. Add or drain oil if necessary. Refer to Procedure 007-043 (Lubricating Oil Level) in Section 3 and Procedure 007-002 (Lubricating Oil and Filters) in Section 4. Check the dipstick calibration. |
| Lubricating Oil Pressure Low | Lubricating oil pressure switch, gauge, or sensor is malfunctioning or is not in the correct location | Check the oil pressure switch, gauge, or sensor for correct operation and location. Refer to the OEM service manual. |
| Lubricating Oil Pressure Low | Lubricating oil does not meet specifications for operating conditions | Change the oil and filters. Refer to Procedure 007-002 (Lubricating Oil and Filters) in Section 4. |
| Lubricating Oil Pressure Low | Lubricating oil temperature is above normal (120° C [248°F]) | Refer to the Coolant Temperature Above Normal symptom tree. |
| Lubricating Oil Sludge in the Crankcase Excessive | Lubricating oil drain interval is excessive | Verify the correct lubricating oil drain interval. Refer to Procedure 102-002 (Maintenance Schedule) in Section 2. |
| Lubricating Oil Sludge in the Crankcase Excessive | Engine is cold | Allow the engine to warm to operating temperature. If the engine will not reach operating temperature, refer to the Coolant Temperature Below Normal symptom tree. |
| Lubricating Oil Sludge in the Crankcase Excessive | Lubricating oil does not meet specifications for operating conditions | Change the oil and filters. Refer to Procedure 007-002 (Lubricating Oil and Filters) in Section 4. Use the oil type recommended in Section V. |
| Lubricating Oil Sludge in the Crankcase Excessive | Bulk oil supply is contaminated with noncontaminated oil | Check the bulk oil supply. Drain the oil and replace with noncontaminated oil. Replace the oil filter(s). |
| Lubricating Oil Temperature Above Specification | Coolant temperature is above specification | Refer to the Coolant Temperature Above Normal symptom tree. |
| Lubricating Oil Temperature Above Specification | Lubricating oil level is above or below specification | Check the oil level. Add or drain oil, if necessary. Refer to Procedure 007-043 (Lubricating Oil Level) in Section 3 and Procedure 007-002 (Lubricating Oil and Filters) in Section 4. |
| Lubricating Oil Temperature Above Specification | Lubricating oil pressure sensor or gauge is malfunctioning or is not in the correct location | Check the oil pressure switch or gauge for correct operation and location. Refer to the OEM service manual for proper operation and the OEM installation instructions for proper location of pressure gauge or sensor. |
Fault codes and symptom→remedy steps extracted from this model’s service/owner documentation; verify against the official manual before servicing.
Documented causes: Load is excessive for engine horsepower rating; Tachometer is not calibrated or is malfunctioning; Air intake or exhaust leaks; Air intake system restriction is above specification. 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. See the table for the matching fixes.
Documented causes: Verify the complaint; Load is excessive for engine horsepower rating; Engine is misfiring; Air intake or exhaust leaks. See the table for the matching fixes.