Cummins/Onan G-855 troubleshooting: 20 symptom-cause-remedy entries from the manufacturer's own service documentation.
Cummins/Onan G-855 — full specs, parts & maintenance.
All Cummins/Onan fault codes →
| Symptom | Cause | Remedy |
|---|---|---|
| Hard Starting or No Start — Insufficient or no gas supply | Gas shut-off closed | Check manual valve; if closed, open slowly |
| Hard Starting or No Start — Insufficient or no gas supply | Gas valve open, low gas pressure to Thermac, engine running | Check electric or Sentinel valve with manual re-set. Re-set to run |
| Hard Starting or No Start — Insufficient or no gas supply | Gas valve open, low gas pressure from Thermac, engine running | Check electric valve for proper voltage being supplied to open. If valve fails to open with proper voltage present, solenoid, or complete valve must be replaced |
| Hard Starting or No Start — Insufficient or no gas supply | Low gas pressure. Thermac adjusted full open, engine running | Consult local gas company for gas supply pressure; Request gas company to supply minimum 10 inches H2O if supply line is of recommended size; If supply line is not of recommended size, an increased pressure to maximum of 20 inches of H2O may be required; Pressure test prior to starting engine indicate 10 inches H2O minimum of gas from main line regulator to Thermac; If static pressure minimum of 10 inches of H2O existed, adjust Thermac to 5 inches H2O with engine idling; Request local gas company to remove orifice and spring from main line regulator to check if they are proper size; Inspect supply line for recommended size and to assure no leakage is evident; With the assistance of local gas company, inspect piping valves, regulator and fittings for obstructions |
| Hard Starting or No Start — Insufficient or no gas supply | Bad in-line gas filter | |
| Hard Starting or No Start — Excess gas | Hard starting - rough idle. Excess gas can be detected by odor, indicating a leak or flooding condition | Stop engine, raise supply line gas pressure from main line regulator to 15 inches H2O; Restart engine. Adjust Thermac to provide 5 inches H2O gas pressure. If unable to maintain 5 inches H2O, stop engine, raise static gas pressure in one inch water column increments until 5 inches of H2O Thermac to carburetor gas pressure can be maintained. Do not exceed 20 inches H2O supply line gas pressure with engine stopped. If 5 inches H2O gas pressure cannot be maintained, a supply problem exists. Contact local gas company; Inspect and adjust gas pressure regulator valves; Check Thermac pressure reducing valve for proper operation and controllability; Check Thermac atmospheric vent for gas leakage; Check Impco carburetor air/gas valve diaphragm for leakage at seating area. Warped diaphragm plate; Check gas power mixture adjustment valve for correct setting; Remove carburetor cover and inspect for broken air valve spring |
| Hard Starting or No Start — Insufficient air | Restricted air intake | Remove and inspect air cleaners. Inspect air piping |
| Hard Starting or No Start — Unbalanced compression pressure | Piston ring or valve leakage | Check individual cylinder compression; Check intake and exhaust valve clearance; Check crosshead adjustment |
| Engine Will Not Idle Properly — Uneven firing of cylinders | Weak spark at spark plug | Check Altronic capacitor for proper charging; Check Altronic output voltage; Inspect primary wiring harness for open or shorted wiring; Incorrect Pick-up Block alignment and/or gapped; Check Altronic ground connection for loose connection; Inspect ignition transformers and HT-leads; Check transformer ground connection. Missing magnet on Timing Disc |
| Engine Will Not Idle Properly — Uneven firing of cylinders | Incorrect or defective spark plugs | Check for recommended type; Check spark plug gap; Check for damaged insulators; Check for shorted electrodes |
| Engine Will Not Idle Properly — Uneven firing of cylinders | Defective Hyd. lifter | |
| Engine Will Not Idle Properly — Improper combustion | Incorrect fuel mixture | Consult gas company to determine if approved gas is being supplied; Check pressure adjustment to determine proper operations of pressure controls; Adjust gas power mixture; Inspect carburetor air/gas diaphragm for rupture or broken spring. Bent B; Inspect gas metering valve for obstruction or improper seating; Inspect cylinder heads to be sure head vent holes are plugged; Check carburetor and intake manifold gaskets for leakage; Check carburetor balance of right and left cylinder banks (12 cylinder engine only) |
| Engine Will Not Idle Properly — Unbalanced compression | Piston ring or valve leakage | Check individual cylinder compression; Check intake and exhaust valve clearance |
| Engine Will Not Idle Properly — Incorrect timing | Valve or ignition timing | Check ignition timing; Remove and inspect air cleaner; Bad turbo; Check intake and exhaust valve timing |
| Engine Fails To Operate Properly Under Load Has A Miss Or Vibration — Ignition | Ignition system functioning improperly | Check for proper operation of Altronic; Timing DOC alignment or missing magnet; Check for proper operation of ignition transformers; Check for incorrect or inoperative spark plugs; Adjust timing |
| Engine Fails To Operate Properly Under Load Has A Miss Or Vibration — Engine shakes or vibrations | Incorrect or improperly adjusted fuel system | Check Thermac for proper operation. Check and adjust gas pressure, check carburetor balance on 12 cylinder engines only |
| Engine Fails To Operate Properly Under Load Has A Miss Or Vibration — Insufficient air supply | Insufficient air supply | Check for air intake restriction and adequate supply; Inspect and adjust gas control valves; Check and adjust gas control valves |
| Engine Fails To Operate Properly Under Load Has A Miss Or Vibration — Unbalanced compression | Piston ring or valve leakage | Check and adjust intake and exhaust valve clearance. Check individual cylinder compression |
| Engine Fails To Operate Properly Under Load Has A Miss Or Vibration — Engine speed falls off greatly | Governor not functioning properly; Friction in throttle linkage | Check governor adjustment and wear. Check speed of governor; Check throttle linkage |
| Engine Fails To Operate Properly Under Load Has A Miss Or Vibration — Very bad or no governor action | No orifice fitting; Flooding governor housing | Install gas orifice in Pierce governor feed oil line; Bad turbo |
Fault codes and symptom→remedy steps extracted from this model’s service/owner documentation; verify against the official manual before servicing.
Documented causes: Gas shut-off closed; Gas valve open, low gas pressure to Thermac, engine running; Gas valve open, low gas pressure from Thermac, engine running; Low gas pressure. Thermac adjusted full open, engine running. See the table for the matching fixes.
Documented causes: Weak spark at spark plug; Incorrect or defective spark plugs; Defective Hyd. lifter. See the table for the matching fixes.
Documented causes: Hard starting - rough idle. Excess gas can be detected by odor, indicating a leak or flooding condition. See the table for the matching fixes.