If your Onan RV generator starts and then shuts down, the most common cause is fuel. The generator’s fuel pickup sits higher in the tank than the engine’s, so it can run dry while your fuel gauge still shows fuel. Next, check the oil level, then read the fault code. Cummins says code 36, the most common service code, means the generator stopped without being told to, usually because it ran out of fuel.
Quick checks when an Onan starts then shuts down
- Fuel level. In the Cummins RV Generator Handbook, Cummins explains that the generator’s fuel pickup typically reaches only about three-quarters of the way down the tank. RV makers do this so the generator can’t strand you with an empty tank. Many owners find the generator quits around a quarter tank. Fill up and try again.
- Oil level. Onan generators have a low oil pressure shutdown. Check the dipstick with the generator level and off. Too much oil can also cause problems, so don’t overfill.
- Fuel filter, hoses and fuel pump. A clogged filter, kinked hose or failing pump can starve the engine once it’s running. On gas models in hot weather, a fuel pump in a hot compartment can vapor lock.
- Air filter. A clogged air filter makes the engine run rich and stall.
- Battery. A weak RV house or chassis battery can cause hard starts. If you have an automatic generator start (AGS) system, check its settings too.
- Load. If it only shuts down when the air conditioner kicks on, the generator may be overloaded or the engine may need service. Turn off other loads and try again.
- Old fuel. Gas left sitting for months can varnish the carburetor. This is one of the most common causes on generators that aren’t run often.
How to read Onan fault codes
Most Cummins Onan RV generators flash a fault code on the status light on the control switch after a shutdown. Here’s how Cummins explains it in the RV Generator Handbook:
- The first blinks are a one-digit code. One, two or four blinks mean a specific shutdown. Three blinks mean a “service” fault with a two-digit code.
- Press Stop once to make the two-digit code blink. Count the first group of blinks, then the second group after the pause. Three blinks, a pause, then six blinks is code 36.
- Press Stop again to stop the blinking.
- If the code has stopped blinking (after about five minutes), press Stop three times within five seconds to bring it back.
Write the code down before you restart the generator.
Common Onan fault codes
| Code | What it means | What to check |
|---|---|---|
| 1 | High engine temperature | Blocked cooling air, debris in the compartment, low coolant on liquid-cooled models |
| 2 | Low oil pressure | Oil level, correct oil type |
| 3 | Service fault | Press Stop once to read the two-digit code |
| 4 | Cranked more than 30 seconds without starting | Fuel supply, priming, spark |
| 36 | Engine stopped without a stop command | Fuel level, fuel filter and hoses, fuel pump |
Codes vary by model, so look up anything else in your generator’s operator’s manual. Cummins posts manuals on its RV generator manuals page.
Prevent shutdowns: exercise your generator
Cummins recommends running gasoline RV generators at least 50 percent loaded for two hours once a month, and says one two-hour run is better than several short ones. Running the air conditioner and a few other appliances is an easy way to load it. Regular exercise keeps fresh fuel moving through the carburetor and keeps parts from sticking.
Other maintenance that prevents most shutdowns:
- Change the oil after break-in and at least once a year, or at the interval in your operator’s manual.
- Replace the air filter and spark plug on schedule.
- Use fresh fuel, and add stabilizer before storage. See our guide to winterizing a generator.
- Keep the generator compartment clean and free of nests.
When to call a technician
If the generator still shuts down after you’ve checked fuel, oil, filters and the battery, or you get a code that points to the control board, alternator or ignition, take it to a Cummins service center or a mobile RV tech. Our list of common reasons a generator won’t start covers more basics.
Related guides: Why is My Generator Not Producing Power, Can Generators Overheat and How to Bypass a Low Oil Sensor on a Generator.
FAQ
Why does my Onan generator shut off when the AC turns on?
The air conditioner’s startup surge may be more than the generator can handle, especially at high altitude or in heat, or the engine may be running poorly from old fuel or a dirty carburetor. A soft-start device on the air conditioner often helps.
Why does my Onan generator run with a full tank but not a low one?
The generator’s fuel pickup is set higher in the tank than the engine’s, so it stops drawing fuel well before the tank is empty.
How do I reset an Onan generator after a fault?
Fix the cause, then press Stop and start the generator normally. If it shuts down again with the same code, the problem isn’t fixed.
Where can I find my generator’s hours?
Many Onan generators have an hour meter on the generator itself or on a remote panel inside the RV. Check your operator’s manual for your model.