Onan RV generators: what fails at altitude and what repairs cost

Onan is what is mounted in most motorhomes and a good many fifth wheels, and in the Prescott area it fails for two reasons that layer on top of each other: ordinary storage neglect, the same fuel and battery issues generators have everywhere, and elevation, a factor that simply does not apply to a generator running near sea level.

The altitude problem, in detail

Every internal combustion generator makes less power as elevation increases, because thinner air means less oxygen for combustion. The standard industry derating figure is roughly three to four percent of rated output lost per 1,000 feet of elevation above sea level for gasoline, diesel, and liquid propane units. At Prescott’s roughly 5,400 feet, that puts a generator in the neighborhood of 15 to 20 percent under its sea-level nameplate rating before anything else is wrong with it.

In practice, this shows up as a generator that used to run two rooftop air conditioners without complaint somewhere else and now trips or struggles here running the same load. It is physics, not a fault, and no amount of repair fixes it, because there is nothing broken. What can help is re-jetting or tuning the carburetor for the actual operating elevation, which recovers some of the lost efficiency and lets the unit run cleaner at altitude than one still set up for sea level. That is a service, not a repair for a failure.

What else fails, and in what order

1. Fuel system. Stale fuel left in a carburetor bowl through a season of sitting still varnishes jets and blocks passages here the same as anywhere. This remains the single most common cause of a no-start.
2. Altitude tuning, covered above, and frequently mistaken for a fault when the real issue is output loss rather than a failed part.
3. Corrosion on connectors and grounds, from winter moisture and condensation in an unheated generator compartment over a season of sitting.
4. Plugs and air filters, neglected because the generator is out of sight and easy to forget.
5. No output with the engine running fine: brushes, capacitor, voltage regulator, or a tripped breaker on the unit itself.
6. Starting and control faults, including the remote start panel and its wiring.

What things cost

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

The habit that prevents nearly all of it

Run it for two hours a month under load. Not at idle, not for five minutes. Under load, meaning with an air conditioner or another real draw running, long enough to get everything hot. Owners who do this essentially never call us about a generator that will not start. It keeps fresh fuel moving through the bowl, circulates oil, and exercises the start circuit.

Setting expectations honestly

If your generator has always struggled to carry two rooftop air conditioners here, that may simply be elevation rather than a fault, and we will say so instead of selling a rebuild that will not change the physics. 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 and are the more effective answer to a marginal generator than a repair.

The safety paragraph

Two things we will always raise: exhaust integrity, since a leaking generator exhaust is a carbon monoxide risk and worth checking on any unit that has not been inspected in a while, and your CO alarm, which should be tested and checked for its expiration date. Both apply everywhere, and neither is optional.

The generator repair service page covers the full scope of what we do on site, across Prescott, Prescott Valley, Chino Valley, Dewey-Humboldt, Williamson Valley, Paulden, and Mayer.