Cummins/Onan YB series troubleshooting: 26 fault codes and symptom-cause-remedy entries from the manufacturer's own service documentation.
Cummins/Onan YB series — full specs, parts & maintenance.
All Cummins/Onan fault codes →
| Code | Meaning | Remedy |
|---|---|---|
| CB1 | Circuit Breaker CB1 | |
| VR1 | Voltage Regulator VR1 | |
| CB21 | Exciter Circuit Breaker (CB21) | Check continuity between VR21-1 and T7, apply shorting jumper if no continuity. Replace circuit breaker CB21 if necessary. |
| VR21 | Voltage Regulator (VR21) | Check continuity between VR21-1 and T7, apply shorting jumper if no continuity. Replace circuit breaker CB21 if necessary. |
| CR21 | Rectifier Bridge (CR21) | Check continuity between VR21-1 and T7, apply shorting jumper if no continuity. Replace circuit breaker CB21 if necessary. |
| T21 | Reference Transformer (T21) | Check continuity between VR21-1 and T7, apply shorting jumper if no continuity. Replace circuit breaker CB21 if necessary. |
| CR1 | Rotating Diode (CR1) | Check continuity between VR21-1 and T7, apply shorting jumper if no continuity. Replace circuit breaker CB21 if necessary. |
| CR2 | Rotating Diode (CR2) | Check continuity between VR21-1 and T7, apply shorting jumper if no continuity. Replace circuit breaker CB21 if necessary. |
| CR3 | Rotating Diode (CR3) | Check continuity between VR21-1 and T7, apply shorting jumper if no continuity. Replace circuit breaker CB21 if necessary. |
| CR4 | Silicon Controlled Rectifier (SCR) 4 | Check continuity between VR21-1 and T7, apply shorting jumper if no continuity. Replace circuit breaker CB21 if necessary. |
| CRS | Silicon Controlled Rectifier (SCR) S | Check continuity between VR21-1 and T7, apply shorting jumper if no continuity. Replace circuit breaker CB21 if necessary. |
| DS12 | Emergency Shutdown | |
| DS13 | Low Oil Pressure | |
| DS14 | High Water Temperature | |
| DS15 | Overspeed | |
| E | ENGINE MALFUNCTION SHUTDOWN | |
| F | LOW OIL PRESSURE SHUTDOWN | |
| G | HIGH WATER TEMPERATURE SHUTDOWN | |
| H | OVERSPEED CONDITION | |
| A11-5 | Overcrank | Check loss of fuel or ignition failure |
| A11-4 | Overspeed | Check engine manufacturer's manual for troubleshooting guide |
| Symptom | Cause | Remedy |
|---|---|---|
| Starter will not crank. | Possible fault location; Fault in Remote station or circuit to station. Starter relay not pulling in because— a. Starter relay K13 faulty. Ai2 printed circuit board faulty. All printed circuit board faulty. Relay K12 faulty. Overspeed switch stuck closed. Short to ground on overspeed switch wire. Starter fauity. | 1. Switch R-S-R- switch to Run. If starter cranks, problem is in Remote circuitry. Refer to troubleshooting guide condition B for this malfunction. 2. If starter does not crank with R-S-R- switch in Run, switch to Stop and check the following— a. Batteries for condition. Connections for security. b. Wiring to starter, and starter solenoid. Secure, clean, etc. 3. Open control cabinet. Check wiring to K13 starter solenoid; K12 Ignition relay; A11 Engine Monitor; A12 cranking control boards. 4. Toggle R-S-R- switch between Run and Stop positions. Action of K12 relay can be observed if it is operating, K13 can be felt. Both relays can be heard. 5. Relays K11 and K12 are interchangeable. If K12 does not operate remove it and plug K11 in its place. If the 'now' K12 relay operates then obviously the old one is defective. install new relay. |
| Low oil pressure shutdown. | This type of malfunction is caused by one of the following— 1. Low oil quantity resulting in low oil pressure. Engine oil system malfunction. Ground between TB11-30 and LOP switch. Permanently closed LOP switch. Faulty engine monitor board. | If the generator set has LOP pre-alarm, then the most possible reason for shutdown is either low oil quantity or an engine oil system malfunction. If the latter, refer to the engine manufacturer's manual for oil system troubleshooting guide. A grounded wire or permanently closed switch are unlikely because the pre-alarm switch has to operate to arm the shutdown system. However, a double fault is possible and should never be discounted. When all other troubleshooting possibilities have been tried, the double fault should be considered. This could be either— 1. Grounded wire between 1TB11-30 and LOP switch. 2. Grounded wire between TB11-34 and LOP pre-alarm switch. 3. Permanently closed LOP switch. 4. Permanently closed LOP pre-alarm switch. 5. Faulty engine monitor board. |
| No generator output | Is panel meter selection switch positioned correctly for a voltage/current reading? - Run 2 Is panel circuit breaker (CB1) in the ON position? - 3 3 Switch CB1 to ON. Does generator output build up? a. Is Output high or low? (See Guide B) b. Is output unstable? (See Guide C) c. Does CB1 trip? (See Guide D) | |
| Generator output too high or too low | Rotate voltmeter/ammeter phase selector switch. Is discrepancy identifiable on all switch positions? - 4 2 Can condition be corrected by adjustment of voltage control rheostat R1 on panel? - 3 3 If output too low, is unit overloaded? - 5 | |
| NO AC OUTPUT VOLTAGE | ENGINE RUNNING RATED RPM | Is the Exciter Circuit Breaker (CB21) on the meter panel in the “ON” position? Switch CB21 to the “ON” position. Is the AC voltage measured at terminals 1 and 2 on voltage regulator (VR21) printed circuit board 5 to 10 volts? Is the AC voltage measured at terminals 11 and 12 on VR21 5 to 10 volts? Replace reactor assembly (L21). Is the DC voltage measured at terminals + and - on Rectifier Bridge (CR21) 5 to 10 volts? Shut down generator set. Check continuity between terminal 2 on VR21 and T8 on generator, between terminal 1 on VR21 and T7 on generator. If no continuity exists between VR21-1 and T7 (CB21-ON) apply a shorting jumper across CB21. Is continuity obtained? Replace circuit breaker CB21. Check for loose or broken wires on VR21, CR21, reference voltage transformer (T21), generator bus-bars and terminal board (TB21) in control box. Secure or repair where necessary. If repairs have been made, restart engine. Are diodes CR1, CR2 and CR3 on CR21 assembly good? Are SCRs 4 and 5 in CR21 good? Fault probably lies with a defective component on VR21. Replace voltage regulator VR21. Start engine. Place CB21 in “OFF” position. Using method prescribed under “M” in procedure section flash the exciter field to restore residual magnetism. Place CB21 ON. Shut off engine. Is the exciter field winding OK? Are the rotating diodes CR1, through CR6 on the exciter rotor OK? |
Fault codes and symptom→remedy steps extracted from this model’s service/owner documentation; verify against the official manual before servicing.
Documented causes: Possible fault location; Fault in Remote station or circuit to station. Starter relay not pulling in because— a. Starter relay K13 faulty. Ai2 printed circuit board faulty. All printed circuit board faulty. Relay K12 faulty. Overspeed switch stuck closed. Short to ground on overspeed switch wire. Starter fauity.. See the table for the matching fixes.
Documented causes: This type of malfunction is caused by one of the following— 1. Low oil quantity resulting in low oil pressure. Engine oil system malfunction. Ground between TB11-30 and LOP switch. Permanently closed LOP switch. Faulty engine monitor board.. See the table for the matching fixes.
Documented causes: ENGINE RUNNING RATED RPM. See the table for the matching fixes.