Onan RV generators: what fails in Arizona and what repairs cost

Onan is what is mounted in most motorhomes and a good many fifth wheels, and in Arizona it fails in one dominant way: it sits. The seasonal use pattern here, run hard from October to April then parked in extreme heat for five months with fuel in the system, is close to the worst thing you can do to a small gasoline engine. Most October and November generator failures trace back to it.

The fuel problem, in detail

Modern pump gasoline is not a storage fluid. In a carburetor bowl at 115 degrees, the volatile fractions evaporate and leave behind a sticky varnish that sets in the jets and the small passages. Those passages are measured in thousandths of an inch. Ethanol content makes it worse in a second way, by drawing moisture that corrodes aluminium from the inside.

So the engine spins over and never catches, or catches and quits, or keeps running but hunts up and down. All three are the same underlying fault and all three are fixed the same way: the carburetor comes off, gets cleaned or rebuilt, and the fuel system gets checked back to the tank.

What things cost

Same approach as everything on the pricing page: diagnose, quote, get your approval, then work.

What fails on these units, most common first

1. Fuel system, far ahead of anything else, because of the varnish described above.
2. Plugs and air filters, neglected because the generator is out of sight.
3. Oil, which is due by hours on the meter and on a lot of East Valley rigs simply never gets changed. Generator hour meters exist for a reason.
4. No AC output while the engine purrs along. That points at the generating end: brushes, a capacitor, the voltage regulator, or the breaker on the generator itself having tripped. Better news than it sounds, since the engine is healthy.
5. Starting and control faults, the remote start switch inside the coach and the wiring back to it included.
6. Exhaust, which is a safety item rather than a performance one and is covered below.

The habit that prevents nearly all of it

Two hours a month, under load. Idling it does not count, and neither does a five-minute start. Under load, meaning with an air conditioner or a heater running, long enough to get everything hot.

Owners who do this essentially never need a call-out for a generator that will not start. It keeps fresh fuel in the bowl, keeps the carburetor wet, circulates oil, keeps the windings dry, and exercises the start circuit. It costs nothing and needs no skill, and no other habit we recommend to Mesa owners pays back like it.

If the rig genuinely has to sit, the next best options are treating the fuel with a stabiliser and running it through, or shutting off the fuel and running the bowl dry where the unit allows it.

Heat, altitude, and honest expectations

Generators are rated under standard conditions, and output falls as temperature rises. On a 110-degree East Valley afternoon your unit is not making its nameplate number. If it has always struggled to carry two rooftop air conditioners in July, that may be physics rather than a fault, and we will tell you so instead of selling you a rebuild.

The practical fixes for that are cheap: stagger the AC starts rather than letting both compressors hit at once, and consider soft-start devices on the air conditioners, which dramatically cut inrush current. Plenty of rigs we see with an “undersized generator” just needed that, for far less than a repair.

The safety paragraph

Two things worth raising on every generator call:

Exhaust integrity. A leaking generator exhaust on a motorhome is a carbon monoxide risk, and these systems live in a harsh environment underneath a vehicle. If one turns up, we tell you, and our recommendation is not to run the unit until it is repaired.

Your CO alarm. Test it, and look for the expiry date printed on it, because these alarms age out. Of everything on this page, that free check matters most.

The generator repair service page covers the full scope of on-site generator work across Mesa, Apache Junction, Gilbert, Tempe, Chandler, Gold Canyon, Scottsdale, and Casa Grande.