Most likely causes (in order of likelihood)

  1. Low oil shutdown. Onan will not crank or will crank but not fire if the oil sensor reads low. This is the #1 cause people miss.
  2. Weak or low house/chassis battery. Generators need solid 12V DC to crank — a battery that starts a vehicle may still be too weak.
  3. Varnished carburetor from sitting. The #1 cause on RVs that haven't been exercised monthly.
  4. Stale fuel. Gasoline starts breaking down at 30 days. After 6 months it can be unusable.
  5. Clogged fuel filter or failed fuel pump.
  6. Bad spark plug or fouled ignition components.
  7. Faulty start solenoid or wiring issue at the controller.

Diagnostic steps (in order, free/cheap before expensive)

  1. Check the oil. Pull the dipstick. If it's below full, top off with the correct weight — Onan is strict about this.
  2. Check chassis battery voltage. With a multimeter, you want 12.6V+ at rest, and it must hold above 9.6V during cranking. If it drops below, replace or charge the battery.
  3. Try priming. On most Onan models, hold the start switch in the STOP/PRIME position for 30 seconds before attempting to start. This pulls fuel through the line.
  4. Listen during crank. If it cranks but never fires, you have a fuel or spark problem. If it doesn't crank at all, you have an electrical or oil-shutdown problem.
  5. Pull the spark plug and inspect. Wet plug = flooded (let it sit). Dry plug = no fuel reaching cylinder. Fouled plug = replace it.
  6. If the generator sat for more than 60 days without exercise and won't start after priming, expect carb cleaning or rebuild. This is the most common 'won't start' fix on stored RVs.

DIY vs. call a tech

Oil check, battery test, priming, and spark plug inspection are all DIY. Carburetor cleaning is intermediate DIY — doable with patience and a service manual. Solenoid replacement and ignition coil work are typically tech-level on Onan units due to access.

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