The 30-second answer: Cummins Onan generators are the standard in RVs and standby home units, and most complaints trace to the fuel or ignition side rather than the generator’s electrical output. Won’t start (RV models): check the start battery and the fuel level lockout — Onan RV gensets won’t start below roughly 1/4 tank on some RVs by design. No AC output with engine running: check the transfer switch, not the generator, first. Runs rough or surges: stale fuel and a gummed carburetor, same as any small engine.
Cummins Onan built its reputation on RV generators (the Onan QG and HGJAB/HGJAA series) and home standby units, so the common issues split between RV-specific quirks and standard small-engine problems.
Won’t start on an RV
Before assuming a generator fault, check two RV-specific things: the coach battery (not just the generator’s own start battery, since some models draw start power from the house battery bank) and the fuel level. Many Onan RV generators have a built-in low-fuel lockout that prevents starting below about a quarter tank, specifically to protect the RV’s main fuel supply — so a generator that “won’t start” with a low gas gauge may be working exactly as designed, not broken.
No AC output with the engine running
On RVs, this is very often the automatic transfer switch, not the generator itself — the ATS routes power between shore power and generator power, and a failed switch leaves you with a running generator and dark outlets. Check the ATS relay and connections before pulling the generator apart; see our transfer switch troubleshooting guide for the diagnostic path.
Runs rough, surges, or won’t hold idle
Standard small-engine carburetor problem — stale fuel gums the idle circuit exactly like it does on any portable generator, and Onan units aren’t immune just because they’re a premium brand. Fresh fuel and a carb cleaning solve the large majority of these cases; see our carburetor cleaning guide for the teardown steps.
Won’t crank at all
Check the start battery’s actual voltage under cranking load first — Onan RV units are notorious for a battery that reads fine at rest but sags badly under real cranking load, especially after a winter of sitting in storage. If the battery checks out, a stuck or corroded start solenoid is the next most common cause.