The generator cranks and won’t catch, or it runs for a while and then dies with the air conditioner going. Before you call anyone, look at the light on the start-stop switch and count the blinks. Two blinks means low oil. Four means it cranked too long without starting, which is usually fuel or battery. Three means a two-digit code is waiting, and pressing Stop once makes it blink out. That number decides most of what happens next. Some codes are a dipstick and a quart of oil. Some mean you are asking the set for more power than the day allows. And some the manual hands straight to a dealer. This guide covers Cummins Onan sets, the built-in generator on many motorhomes and some trailers, because their manuals write every one of those down.
What do the blinking lights on my Onan generator mean?
The light on the switch is the generator talking to you, and it only has a few words.
Two blinks is low oil pressure. Four blinks is an overcrank: the starter ran for 15 to 20 seconds on the smaller sets, 30 seconds on the bigger HGJAB sets, and the engine never caught. Three blinks is a service fault, and that one has a second layer. Press Stop once and the light blinks out a two-digit code.
Write the number down before you do anything else. Once the fault clears, it’s gone, and it’s the single most useful thing you can hand a technician.
Here’s how the manual sorts the codes you’re most likely to see:
| Code | What it means | Yours or a tech’s |
|---|---|---|
| 2 | Low oil level | Yours. Check the dipstick |
| 13, 15 | Undervoltage, underfrequency | Yours. Fewer appliances running |
| 22, 38 | Governor actuator overload, field overload | Yours first. Shed load, then check the air cleaner, the exhaust and the AC |
| 32 | Low cranking speed | Yours. Battery and cables |
| 36 | Engine stopped without command | Yours first. Fuel, plugs, air cleaner |
| 57 | Overprime | Yours. You held Stop/Prime too long |
| 12, 14, 19, 27, 41, 45, 48, 51 | Voltage regulator, governor, sensing and processor faults | A tech. The manual says see an authorized Onan dealer |
That last row is the line Cummins draws for you. There’s nothing an owner is meant to do about those codes except write them down and make the call.
Why won’t my RV generator start?
If it cranks and won’t catch, or barely turns over, work these in order.
Fuel the generator can actually reach. This is the one that fools people. When the generator shares a fuel tank with the engine, its pickup tube sits higher. Cummins puts it plainly: “The genset fuel pickup tube is probably higher up in the fuel tank than the vehicle engine pickup.” So the gauge says a quarter tank and the generator is out of fuel. The manual’s suggestion is to mark the point on your fuel gauge where the generator quits, so you know where its empty is.
Prime it after running dry. Once it’s sucked air, hold the switch at Stop/Prime for 30 seconds to refill the fuel system, then try again. Don’t hold it much longer than that. Past three minutes the set throws code 57 for overprime.
The battery. Slow cranking is code 32, and the fix is at the battery: clean and tighten both cables, at the battery and at the generator, then recharge or replace the battery. The manual adds a detail that matters on a cold morning: thick oil slows cranking too.
Propane in the cold. On an LP generator, a cold tank can’t turn liquid into gas fast enough to keep up. Cummins says that on cold days the container may need to stay at least half full to supply the generator. A set that starts on a mild afternoon and won’t on a frosty morning, with the tank low, is a likely case of this.
Why does my RV generator start and then shut down?
Runs for a while, then quits. Two families of cause, and the code tells you which.
It ran out of something. Code 36 means the engine stopped on its own. The manual’s owner list starts with fuel, then the spark plug leads, the plugs themselves, the air cleaner, and a look for anything mechanically wrong. If it dies after the same stretch every time, restarts once it cools, and dies again, that pattern is worth telling a tech about along with the code.
Low oil. Code 2 is the oil level, and it can be wrong in either direction. Low, add oil. Overfilled past the Full mark, drain some out. Cummins wants the oil level checked every day or every 8 hours of running, and the oil and filter changed every 50 hours. The smaller-set manual adds a line worth reading twice: “Change oil more often in hot and dusty environments.”
You asked it for too much. This is the one people mistake for a broken generator. The set starts, runs fine with the lights and the fridge, and drops out the moment the second air conditioner kicks on. Codes 13, 15, 22 and 38 all get the same answer from Cummins: “Reduce the number of connected appliances, especially when air conditioners and battery chargers are running.” Code 22 also points at a dirty air cleaner or a blocked exhaust. Code 38 adds one more thing to check. A failing air conditioner can do this on its own, and the manual says a locked compressor can cause very low power factor. If the generator only quits when one particular AC unit starts, have that unit looked at. Our AC not cooling guide covers what to check on that side.
Why does my generator struggle in the heat or up in the mountains?
Every generator has a rated output, and the rating assumes cool air near sea level. Move away from either one and you have less to work with, before you switch anything on. This is the part nobody reads until the set starts quitting at three in the afternoon.
Heat. Cummins prints the number. Rated engine power drops about 1 percent for every 10 degrees F above 77. On a 107-degree afternoon that’s 3 percent gone. It doesn’t sound like much, until you remember the air conditioners are working hardest at the exact moment the generator has the least to give them. The manual says as much: “It may be necessary to run fewer appliances at higher altitudes and ambient temperatures.”
Altitude. Thin air takes more. Cummins puts the loss at about 3.5 percent of rated power for every 1,000 feet you climb. The smaller-set manual prints a table for its 4,000-watt generator:
| Elevation | What a 4,000-watt set can deliver |
|---|---|
| Up to 500 feet | 4,000 watts |
| 2,500 feet | 3,720 watts |
| 5,500 feet | 3,300 watts |
| Above 5,500 feet | 140 watts less for every further 1,000 feet |
To put that on a map: in northern Arizona, Flagstaff sits at about 7,000 feet. A 4,000-watt set parked there is working with less than 3,300 watts before the afternoon heat takes its share. A generator that ran two air conditioners near sea level can struggle to carry one at that height, and nothing is wrong with it.
The carburetor setting. Some sets have to be told what altitude they’re at. For the HGJAB series, Cummins says to “reset the carburetor for the current altitude” anywhere between 0 and 5,000 feet. The trap runs in both directions, and the downhill one is worse: “Operating the generator set at low altitude with a high altitude setting can cause power loss, overheating and engine damage.” If you spent the summer in the mountains and came back down, check that setting before you blame anything else. If you’re staying above 5,000 feet for a long stretch, Cummins says the set needs a high altitude kit.
Your own manual has the exact numbers for your model. The rule is the same for all of them: hotter and higher means fewer appliances.
Run it every month, or it won’t start when you need it
The maintenance rule the manual repeats most is also the easiest one to skip. “Exercise the genset at least 2 hours each month if use is infrequent. Run the genset at approximately 1/2 rated power.”
Half load matters. Put a real load on it, about half of what it’s rated for, rather than letting it idle with nothing plugged in. And Cummins says one two-hour run beats several short ones. The reason is that a generator sitting still goes stale from the inside: moisture collects, oil drains off the engine parts, fuel goes old in the lines and carburetor, and the electrical contacts oxidize. A proper run drives all of that off.
A rig that sat in storage all season without this is the one that won’t start on the first night you need it. Exercise it before the trip, at home, where a failed start is an inconvenience instead of a hot night.
Carbon monoxide: the part that isn’t a repair
If you run the generator while you sleep, for the air conditioner, read this section twice.
Cummins says never to be inside the vehicle with the generator running unless it has a working carbon monoxide detector. It wants the exhaust checked for leaks at every startup and after every eight hours of running. And if there’s a leak, or any chance exhaust is getting into the rig, don’t run the generator at all.
Check the detector’s date before the season starts, not halfway through it. And look at where the exhaust ends up when you park. If it can drift back into the rig, Cummins says the generator stays off.
Should I fix it myself or call a tech?
Here’s the honest split.
Yours, today, with no special tools:
- Reading the blink code
- Checking and topping up the oil, or draining an overfill
- Getting fuel to the generator and priming it
- Cleaning and tightening battery cables, charging the battery
- Turning off appliances to get under the load limit
- Resetting the altitude setting, if your manual shows you how
- The monthly exercise run
A tech’s: anything the code table sends to a dealer, plus carburetor rebuilds, stator and brush problems, and the control board. Cummins is direct about it: “Only trained and experienced service personnel with knowledge of fuels, electricity, and machinery hazards should perform genset service.” If the set is under warranty, ask whether the tech is Onan authorized before they touch it.
A mobile tech can do most generator work right in the generator bay at your site. What makes the visit short is what you tell them in the first minute:
- The model, off the nameplate behind the access door
- The blink code, and the two-digit code if there was one
- The reading on the hour meter
- When it quits: at startup, under load, after a set time, or only in the afternoon heat
Your next step right now: check the oil, look at the fuel gauge against where the generator’s empty point is, and start it with everything else switched off. If it runs clean with no load and quits when you add the air conditioner, you’ve found a load problem, not a broken generator. If it throws a code the table gives to a dealer, write it down and call one of the techs below. They say generators are among the systems they work on.