Onan RV generators report problems with a blinking status light on the control switch. One-digit codes blink once in a group: 2 blinks means low oil pressure, 3 means a service code is stored, and 4 means overcrank (it cranked 30 seconds without starting). For a 3-blink code, press STOP/PRIME once to show the two-digit code, such as 36 (engine stopped on its own) or 32 (low cranking speed). The list below comes from Cummins’ operator’s manual for the Onan HGJBB (QG 2800) generator. Codes are similar across Cummins Onan RV models, but check your own manual, because a few codes differ by model.
How to read Onan blink codes
- Single-digit codes (2, 3, 4) blink as one group, then pause and repeat. Count the blinks.
- Code 3 means “look deeper.” Press STOP/PRIME once and the light flashes a two-digit code.
- Two-digit codes blink the tens digit, pause briefly, then blink the ones digit. Code 36 is 3 blinks, a short pause, then 6 blinks. A longer pause separates repeats.
- How long the code lasts: the last fault keeps flashing even after you fix the problem, and stops after 5 minutes. To see it again, press STOP/PRIME three times within 5 seconds.
Source: Cummins Onan HGJBB operator’s manual (A031C171).
Onan generator fault code list
| Code | Meaning | What to do |
|---|---|---|
| 2 | Low oil pressure | Check the oil level. Add oil if low, or drain some if it’s overfilled. |
| 3 | Service check | Press STOP/PRIME to read the two-digit code. |
| 4 | Overcrank (cranked 30 seconds without starting) | Check fuel (keep LP at least half full in cold weather), prime the fuel system, check the spark plug, air filter, fuel filter and oil grade. |
| 12 | Over voltage | See a Cummins dealer. |
| 13 | Under voltage | Reduce the load, especially air conditioners and battery chargers. |
| 14 | Over frequency | See a Cummins dealer. |
| 15 | Under frequency | Reduce the load. |
| 27 | Voltage sense | See a Cummins dealer. |
| 29 | High battery voltage (over 19 volts) | Make sure 12-volt batteries are wired in parallel, not series, and lower the battery charge rate. |
| 31 | Overspeed (over 3,400 RPM) | See a Cummins dealer. |
| 32 | Low cranking speed | Clean and tighten battery cables, charge or replace the battery, check the oil grade. |
| 35 | Control card failure | See a Cummins dealer. |
| 36 | Engine stopped without a command | Check fuel or LP supply and prime, check the spark plug and air filter, look for mechanical damage. |
| 37 | Invalid generator configuration | See a Cummins dealer. |
| 38 | Over current (field overload) | Reduce the load and have air conditioners and appliances checked. |
| 41 | Generator rotor | See a Cummins dealer. |
| 42, 43 | Processor error | See a Cummins dealer. |
| 45 | Speed sense | See a Cummins dealer. |
| 47 | Ignition | See a Cummins dealer. |
| 48 | Generator field sense | See a Cummins dealer. |
The most common Onan codes
Code 36: engine stopped
The engine quit without the control telling it to. Cummins lists fuel first: check the gas or LP supply and prime the system. In many RVs the generator’s fuel pickup sits higher in the tank than the engine’s, so the generator can run dry before the vehicle tank is empty. If yours starts and then quits, see why an Onan generator starts then shuts down.
Code 4: overcrank
It cranked for 30 seconds and never caught. Prime the fuel system (see your manual for how long to hold STOP/PRIME), then try again. Check fuel first, then the spark plug and air filter. Here are common reasons a generator won’t start.
Code 2: low oil pressure
Check the dipstick with the RV parked level. Too little or too much oil can trigger it. Here’s what oil a generator uses.
Codes 13, 15 and 38: overload
The generator can’t keep up with the load. Turn off one air conditioner, the water heater or the battery charger, then restart. If you need to run more, see what size generator you need.
Onan diesel generator codes
Onan Quiet Diesel models (such as the 7.5 and 10 kW QD) use the same blink-code system, but some code numbers and meanings differ by model and year. Find your model and spec number on the generator’s nameplate, then use the fault code table in that model’s operator’s manual from Cummins.
FAQ
How do I clear an Onan fault code?
Fix the cause, then start the generator normally. The last code keeps flashing for 5 minutes after a fault even once it’s fixed; that’s normal.
What does 3 blinks mean on an Onan generator?
A service code is stored. Press STOP/PRIME once to see the two-digit code.
Why does my Onan show the same code after I fixed it?
The last logged fault keeps flashing for up to 5 minutes after a shutdown, even after the cause is fixed. Start the generator normally; if it runs, the problem is solved.