Generator Inspection: Hours, Load Test, and Service History

An onboard RV generator is a small, neglected engine that lives a hard life, so inspect it like a used car, not an accessory. Read the hour meter, start it cold, then put it under a real load (run two air conditioners or a space heater) and watch for surging, smoke, or a unit that simply will not carry the load. A generator with documented oil changes and 500 hours can outlast a "low hour" unit that sat untouched for six years, so service history and how it ran matter more than the number on the meter.

Most used RV listings treat the generator as a checkbox: "Onan generator, runs great." That sentence is doing a lot of work, and it is usually the seller's word against your wallet. A replacement RV generator commonly runs $1,200 to $5,500 installed depending on the type and output, so a bad one is not a footnote. This is one of the most expensive single components on a motorhome that almost nobody tests correctly before they buy.

Below is exactly how to inspect it, what the hours actually mean, how to run a load test even if you have never touched a generator, and the dollar ranges and walk-away lines that keep you from inheriting somebody else's problem. When you are done, paste the listing and get your RV verdict so the generator condition gets weighed against everything else on the unit.

What kind of generator are you even looking at?

Before hours or load tests, figure out what is installed. The inspection and the cost of being wrong change a lot by type.

• Onboard gas or LP generator (Onan/Cummins, Generac): Standard on most Class A motorhomes and many Class C motorhomes. Runs off the RV's main fuel tank (gas or diesel) or a dedicated LP supply. This is the unit this article is mostly about.
• Onboard diesel generator: Common on larger diesel-pusher Class A coaches. Tougher and longer-lived, but parts and service are pricier.
• Portable inverter generator (Honda, Champion, Predator): What most travel trailer and fifth wheel owners use instead of an onboard unit. Easier to inspect, easier to replace, and it leaves with the seller unless it is explicitly in writing as included.

If the listing says "generator" on a travel trailer, ask whether it is a built-in unit or a portable that may or may not convey. Get the answer in the message thread, not over the phone.

Reading the hour meter (and why "low hours" can be a trap)

Onboard generators usually have an hour meter, sometimes on the generator itself, sometimes on the RV's system display. Find it and write the number down. Then resist the urge to treat a low number as good news.

A typical onboard RV generator is built for somewhere in the range of 3,000 to 10,000+ hours of life with maintenance, with diesel units on the higher end. So 600 or 1,200 hours on a 10-year-old coach is not "worn out." Here is the trap: generators hate sitting more than they hate running. A unit with 150 hours that sat for five years with stale fuel, gummed carburetor jets, and dried-out seals can be in worse shape than a 2,000-hour unit that ran monthly.

What the number actually tells you:

Do the math against the RV's age. Roughly 50 to 150 hours per year is normal. A 12-year-old coach showing 80 hours is a generator that has mostly napped, which is its own kind of risk. Ask the seller directly: "How often did you run the generator under load, and when did you last run it?" Hesitation is information. This pattern lives alongside the other big number game on used RVs, how old the tires really are, where "looks new" and "is safe" are not the same thing.

For a deeper breakdown of how to interpret the meter against price, see understanding RV generator hours.

The cold start: what to watch in the first 90 seconds

Insist on a cold start. If the seller has already warmed it up "to save time," that is a polite red flag. A warm generator hides the exact problems you came to find. Ask them not to touch it until you arrive, or to let it sit at least a couple of hours.

Stand at the generator (or its exhaust) and watch the first start sequence:

1. Cranking: It should crank and catch within a few seconds. Long cranking, multiple attempts, or needing throttle tricks suggests fuel delivery or carb issues.
2. First exhaust: A brief puff is normal. Persistent blue smoke means burning oil (worn rings or valve seals). Persistent black smoke means it is running rich (carb, choke, or fuel mixture). Thick white smoke after warmup can mean coolant or a head-gasket issue on liquid-cooled units.
3. Idle and run: Within 30 to 60 seconds it should settle into a steady hum. Surging (rhythmic rise and fall in engine speed) is the classic sign of fuel or carburetor trouble on a unit that has sat with old fuel.
4. Smell and sound: Sharp fuel smell, knocking, or rattling are all "pause the inspection" signals.

Let it run with no load for a few minutes and confirm it stays steady. If it dies, surges, or never smooths out with zero load, you do not need to go further. That is a unit that needs work, and you price accordingly.

The load test: the step almost everyone skips

A generator that idles fine in the driveway tells you almost nothing. Generators fail under load, which is the one condition you actually need it for. This is the single most valuable five minutes of the whole inspection.

How to load it, in plain terms: turn on the things that draw real power.

• Switch the RV to generator power.
• Turn on the air conditioner, and if it has two, run both. A/C compressors are the heaviest common load and the truest test.
• If it is cold out, run an electric space heater or two plus the microwave to simulate the draw.
• Run the microwave for a minute on top of the A/C.

A healthy generator takes the load with a brief dip in engine speed, then holds steady. Watch and listen for:

• Bogging down or stalling when the A/C compressor kicks in.
• Surging or hunting under load that was not there at idle.
• A/C that will not start or trips on generator power but works on shore power (often a generator output problem).
• Voltage sag: if you have a simple plug-in voltage meter or a multimeter, output should sit near 110 to 125 volts under load. Sustained readings well below that mean it cannot carry the load.

Let it carry the load for 5 to 10 minutes, not 30 seconds. Some units start fine and then drop off as they heat up. If the seller refuses a load test or claims "you can't run the A/C on the generator," treat that as a likely defect, not a quirk. A working onboard generator is supposed to run the A/C. That is the whole point.

A weak generator under load is also a doorway to broader RV electrical problems, so if the lights flicker hard or the inverter throws faults, widen your inspection.

Service history and the maintenance checklist

Paperwork separates a cared-for generator from a gamble. Ask for receipts, a maintenance log, or even a photo of an oil-change sticker on the unit. No records does not automatically mean no maintenance, but it shifts the risk onto you, and you should price that in.

Walk through this checklist on the actual unit:

• Oil level and condition. Pull the dipstick. Clean amber to light brown is good. Black and thin, or milky/foamy (coolant intrusion), is a problem.
• Oil change history. RV generators typically want an oil change every 100 to 150 hours or annually. Ask when it was last done.
• Air filter. A filthy, oil-soaked, or rodent-nested air filter signals neglect and chewed wiring nearby.
• Spark plug(s). On gas units, fouled plugs hint at running issues.
• Fuel system. Ask if it has stale fuel or has been run dry for storage. Gummed carburetors from old gas are the number one "low hour" failure.
• LP units: confirm clean burn and no propane smell at the regulator.
• Mounting and exhaust. Look for cracked exhaust, rust-through, or loose mounts. Exhaust leaks near a motorhome bedroom are a carbon monoxide risk, not a cosmetic issue.
• Rodent damage. Pop the access panel and look for nests, chewed insulation, and gnawed wiring. Stored generators are mouse condos.
• Annual exercise. Manufacturers recommend running the generator under load about an hour a month. Ask if the seller did.

If you want the full unit-wide version of this, pair it with the used RV inspection checklist so the generator is one line in a complete walkthrough, not your only focus.

What it costs to fix, and what it costs to replace

Knowing the dollars turns a scary unknown into a negotiation. These are typical ranges in North America. Verify on the specific unit and your local labor rates, because generator service shops are not cheap.

Use these as leverage. If the generator surges under load and the seller has no records, a fair ask is to knock the likely repair range off the price or have the seller fix it before closing. Run the whole picture through the used RV offer calculator so the generator condition flows into a number you can defend, and the RV ownership cost calculator if you want to see how generator service fits your yearly budget.

Seller questions and walk-away lines

Ask these directly and watch how they answer:

• "When did you last start it, and can we do a cold start when I arrive?"
• "Can we run both A/C units off the generator while I'm here?"
• "When was the last oil change and do you have any service records?"
• "Has it ever sat for a season without being run? With fuel in it?"
• "Has it ever failed to start or surged on you?"

Walk away, or price for a known repair, if:

• The seller refuses a cold start or a load test, or "just had it serviced" and warmed it up before you got there.
• It surges or stalls the moment the A/C compressor engages and never recovers.
• You see steady blue smoke (oil burn) or milky oil on the dipstick (coolant).
• There is obvious rodent nesting and chewed wiring in the compartment.
• The exhaust is rusted through or leaks near living space (CO risk).
• "Generator included" turns out to be a portable that the seller is keeping.

None of these mean the whole RV is bad. They mean the generator is a separate negotiation, and you now have the facts to lead it. Generator condition is one item on a broader risk map; see the full RV red flags list so you are not so focused on the genset that you miss water damage.

FAQ

How many hours is too many on an RV generator? There is no hard cutoff. Many onboard gas units are built for 3,000 to 10,000+ hours with maintenance, and diesel units often go further. A 2,000-hour unit that was serviced and exercised can be healthier than a 200-hour unit that sat. Judge it by how it runs under load plus service history, not the meter alone.

Can I really run the air conditioner off the onboard generator? Yes, that is exactly what an onboard generator is sized to do on most motorhomes. If a seller tells you the A/C "can't" run on the generator, that usually points to an undersized, weak, or failing unit, and it is grounds to test harder or walk.

The generator starts but surges. Is that a big deal? Usually it is stale fuel and a gummed carburetor, which is common after storage and often a $150 to $600 fix. It is not automatically a deal-breaker, but you should price it as a known repair rather than trust it will "clear up on its own."

Do I need a generator at all? Only if you plan to dry camp (no hookups) or want A/C and microwave power off-grid. Many travel trailer and truck camper owners skip the onboard unit entirely and use a portable inverter generator or a battery and solar setup instead.

Should a generator problem kill the whole deal? Rarely. A generator is a contained, quotable repair. The deals worth walking from are usually structural: soft floors, roof leaks, and hidden water damage. Treat the generator as a line item, get a real number on it, and fold it into your offer.

Paste your RV listing and get your RV verdict before you drive out to look at it.