Onan generator won't start? Here's the order of operations.

Cummins Onan generators are tanks, but they have specific quirks. Most no-start issues come down to fuel level, a dead starter battery, or a fault code you can read yourself.

Before we dig in, figure out which failure mode you're dealing with:

• No crank — You press Start and nothing happens, or you hear a weak click.
• Cranks but won't fire — The starter turns the engine over, but it won't catch and run.
• Starts and immediately dies — Fires up for a second or two then shuts down.
• Displays a fault or blinks a code — The Onan controller is telling you what's wrong.

No crank: the electrical side

1. Check your chassis fuel level

This catches everyone once. On most motorhomes, the Onan generator pulls fuel from the same tank as the chassis, but through a standpipe that sits about 1/4 of the way up the tank. This is a safety feature: you can't run the generator so low that you strand the motorhome. If your fuel gauge is below 1/4 tank, the generator will not run even if there's fuel in the tank.

Add fuel and try again.

2. Check the starting battery

Gas Onan generators typically use the chassis battery to crank. Diesel Onans often use the house batteries. Either way, if the battery is weak, the starter motor will click but not turn the engine.

Start the motorhome engine first (if it's a motorhome), let it run for 5 minutes to charge the battery, then try the generator again.

3. Check the generator circuit breaker

The Onan has its own circuit breaker on the generator itself, usually accessible from the outside compartment door. If the breaker is tripped, reset it. Also check for a blown fuse in the line from the battery to the generator.

Cranks but won't fire: the fuel and air side

4. Prime the fuel system

If the generator has been sitting for months, the fuel lines may have drained. Hold the Start/Stop switch in the PRIME position (usually down) for 30 to 60 seconds before trying to start. This runs the fuel pump and fills the lines. Then try starting normally.

5. Stale fuel

Gasoline goes bad in about 90 days. If the rig has been sitting through a winter, your old fuel may not ignite. If you suspect stale fuel, the fix is to drain it and refill with fresh gas. This usually requires a tech.

6. Air filter check

Find the air filter housing on the generator and pop it open. If the filter looks like a dryer lint trap, that's your problem. A clogged air filter starves the engine of air. Clean or replace it.

Fault codes: let the generator tell you what's wrong

Modern Onan generators blink fault codes on the start panel when they fail. Count the blinks in sequence and look them up:

• 1 blink — High engine temperature. Let it cool, check airflow to the generator compartment.
• 2 blinks — Low oil pressure. Check oil level. This is common and easy to fix, but dangerous to ignore.
• 3 blinks — Overcrank. Engine tried to start but couldn't. Points to fuel, spark, or air.
• 13 blinks — Overvoltage. Something on the AC output side is wrong.
• 14 blinks — Undervoltage. Similar but opposite.
• 19 blinks — Fuel pump fault.
• 32 or 36 blinks — Engine speed / governor issues.

Full fault code list is in your Onan operator manual. The model number is on a sticker on the generator housing.

7. Check the oil level

Onan generators have a low oil shutdown. If the oil is even slightly below the add line, the generator will crank but refuse to start. Pull the dipstick on the generator itself (not the chassis engine). Add oil if needed.

When to call a mobile RV tech

• 2-blink oil pressure code and oil is at the right level
• Generator starts but won't carry load (AC trips breakers immediately)
• You see oil or fuel leaking from the generator compartment
• Generator makes a loud knocking or banging sound
• Exhaust is very black or very white (fuel or coolant problem)

Frequently asked questions

Why won't my Onan generator start?

The top three causes are: low oil shutdown (Onan refuses to start if oil is even slightly low), stale fuel from sitting, and a dead start battery. Check oil first with the dipstick on the side of the gen, then check fuel level (Onan stops drawing fuel when the tank hits 1/4), then load test the start battery.

What is the Onan blink code for low oil?

Most Onan QG and Microquiet generators blink fault code 36 or 37 for low oil pressure. The exact code varies by model but the genset will refuse to crank or shuts down within seconds. Add oil to the full mark on the dipstick and try again. Use 15W-40 in summer, 10W-30 in winter.

Why does my Onan crank but not start?

Cranks-no-start usually means no fuel reaching the carburetor or no spark. Stale gas after sitting all winter is the #1 culprit. Try fresh fuel and run the gen primer (hold the start switch in the prime position for 30-60 seconds). If still nothing, the carburetor likely needs cleaning.

How often should I exercise my Onan generator?

Run it under load (AC on, or a 1500W heater plugged in) for at least 2 hours every month, even in storage. Light running with no load causes wet stacking and carbon buildup. A monthly hour or two of real load keeps the carb clean and the windings dry.

Does my Onan generator have its own fuel tank or share with the chassis?

It shares the chassis fuel tank but has its own pickup tube that's higher than the chassis pickup. This is intentional: the generator stops drawing fuel when the tank hits about 1/4, so you can't strand yourself by running the gen dry. If your gen quit but the dash gauge says 1/4, that's why.