Your RV generator won’t start, cranks and dies, or runs while the outlets stay dead. Most RV generator repair starts with four things: fuel, oil, the starting battery and the breaker on the generator itself. This page explains how an onboard generator works, which parts fail, what the Cummins Onan manuals say, and where DIY work should stop.
How does an RV generator work?
An onboard RV generator is a small engine that spins an alternator to make 120 volt AC power. Many motorhomes carry a Cummins Onan unit that burns gasoline from the RV’s own tank. Others run on diesel or propane. The steps below follow a gasoline Onan.
- Prime. Holding the switch at STOP/PRIME runs the fuel pump for 30 seconds to fill the fuel lines. Cummins says to prime after running out of fuel or changing the fuel filter.
- Crank. Holding START sends 12 volt battery power to the starter, and the status light on the switch flashes. If the engine doesn’t start in about 30 seconds, cranking stops and the light blinks an over crank fault.
- Run. The status light stays on. Cummins asks you to let the engine warm up for 2 minutes before you turn on appliances.
- Watch for faults. The control board checks output voltage, engine speed and, on most models, oil. If a reading goes out of range, it shuts the engine down and blinks a fault code.
- Send power out. Power leaves through the generator’s breaker and goes to the transfer switch, a box that picks generator or shore power. If that breaker trips, the engine keeps running but the RV gets nothing.
Which RV generator parts fail, and what are the signs?
| Part | What it does | Sign it is failing |
|---|---|---|
| Starting battery and cables | Power the control board and starter | Dead status light, slow crank, or a starter that clicks in and out (Cummins says cranking voltage has dipped below 6 volts) |
| Fuel pump and filter | Move fuel to the engine | Cranks but won’t start, or quits with code 4 or 36 |
| Carburetor or fuel injection | Mixes fuel and air | Hard starting and rough running after storage, from gum and varnish in old gas |
| Spark plug and air filter | Fire the fuel and clean intake air | Rough running or a code 36 shutdown |
| Oil pressure or oil level switch | Stops the engine before it runs dry | Starts, then stops with a low oil code |
| Generator breaker | Protects the output | Generator runs, status light on, no power in the RV |
| Transfer switch | Chooses generator or shore power | Breaker on and generator running, but the RV stays dead; buzzing or a hot smell |
What should I check first when my RV generator won’t run?
- Fuel level. When the generator and engine share a tank, Cummins says the pickups are usually set so the generator runs out first. Fill up, then prime for 30 seconds before you crank.
- Oil. Park level, stop the generator, and keep the oil between the dipstick marks.
- Starting battery. Clean and tighten the cables at the battery and the generator, then charge the battery.
- Blink code. Count the blinks on the status light. On many models, 3 blinks means press STOP/PRIME once to see a two-digit code. Look it up in our Onan QG 4000 code list or QG 5500 and 7000 list.
- Breakers. Reset the breaker on the generator, then the one in the RV panel. Push each fully off before on.
- Load. Cummins says to turn off the air conditioner and other large appliances before you start.
Safety first. Generator exhaust carries carbon monoxide (CO), a gas you can’t see or smell that can kill. Cummins says to test your CO detectors and look for exhaust leaks before every start and after every 8 hours of running, and never to stay in the RV while the generator runs without a working CO detector. Do not start it if you smell fuel or see a leak.
RV generator specs to know
These numbers come from the Cummins Onan operator manuals. Your exact model may differ, so check the nameplate and the manual for your generator.
| Spec | Onan QG 4000 (gas) | Onan QG 5500 / 7000 (gas) | Onan QD 6000 / 7500 / 8000 (diesel) | Onan QG 2800i (inverter) |
|---|---|---|---|---|
| Rated output | 4,000 W, 120 V, 33.3 A | 5,500 W (45.8 A) or 7,000 W (58.3 A), 120 V | 6,000, 7,500 or 8,000 W, 120 V | 2,800 W, 120 V, 23.3 A |
| Generator breaker | See your model’s manual | See your model’s manual | 2-pole, 30 A or 35 A | 30 A, plus a built-in “virtual breaker” |
| Engine oil | 1.6 qt (1.5 L) | 2.0 qt (1.8 L) | 3 qt (2.6 L) with filter | 1 qt at an oil change |
| Starting battery minimum | 450 CCA at 0°F | 450 CCA at 0°F | 450 CCA at 0°F, 650 CCA at -20°F | 650 CCA |
| Low oil fault | No low oil code in its fault list | 2 blinks, low oil pressure | 2 blinks, low oil pressure | Code 6, low oil level |
| Exercise if used rarely | 2 hours a month at about half load | 2 hours a month at about half load | 2 hours a month at about half load | 2 hours a month at about half load |
Which RV generators are out there, and how do they differ?
Cummins Onan built-in RV generators come in three families. The QG line burns gasoline, and some models run on propane; some QG 5500 and 7000 models use fuel injection, which adds sensor codes such as 52, 54 and 56. The QD line runs on diesel, is water-cooled, and charges its own starting battery at up to 10 amps. The inverter models (QG 2800i and the 4000i and 7000i dual fuel) use a different code list: on the 2800i, code 6 means low oil and code 38 means the built-in overload protection tripped.
Write down the model and spec letter from the nameplate before you look up a code or order parts, because codes and parts differ by series.
Portable inverter generators work another way: the RV’s shore cord plugs into them, with no transfer switch. If the RV gets no power, check the generator’s outlet, overload protection and oil per its manual.
When should you call an RV tech for generator repair?
- Codes with no owner fix. For codes like 12, 14, 37 and most codes in the 40s, the Onan manuals say to contact a dealer.
- A breaker that trips again right away. Cummins says that points to a short or a faulty breaker, and to call a qualified electrician.
- Code 36 that keeps coming back after you check fuel, spark plug and air filter. The manual’s last step is a check for mechanical damage.
- Fuel or exhaust leaks, knocking, or heavy smoke.
- No power with every breaker on. Testing the alternator or transfer switch means working on live 120 volt parts.
Shops bill by the hour, so doing the cheap checks first means less time on the clock.
Common questions
What are the most common problems with RV generators?
Running out of fuel, low oil, weak starting batteries, stale fuel after storage and overloads cause most shutdowns. Cummins says that keeping oil topped up, keeping battery connections clean and tight, watching the fuel gauge and not overloading the generator prevent most of them.
Why won’t my RV generator start?
Start with fuel: on a shared tank the generator runs dry first, so fill up and prime for 30 seconds. Then check the oil and the starting battery. If it cranks for about 30 seconds and stops with 4 blinks, that is an over crank fault, which Cummins ties to fuel, a loose plug wire, a worn plug or a dirty air filter.
Is there a reset button on an Onan generator?
No. After an over crank fault, wait 5 seconds for the control to reset before you try again, and let the starter cool for 30 seconds after every fifth try. The last fault code keeps flashing even after the problem is fixed; press STOP/PRIME to stop it.
Why does my generator run for a few seconds and then stop?
Low oil and fuel starvation are the usual causes. Code 36 means the engine stopped on its own, and Cummins says to check fuel, prime, then check the spark plug and air filter. Read the blink code before you retry.
Why isn’t my RV generator charging the batteries?
The generator makes 120 volt power, and the RV’s converter uses it to charge the house batteries. Check the generator breaker, give the transfer switch a moment, then check the converter (see RV electrical problems). If the starting battery keeps running down, the QG 4000 manual says to add or service a battery charging system in the vehicle.
What is the average life expectancy of an RV generator?
Cummins does not publish a lifespan in its operator manuals, and we won’t guess one. The manuals do say how to care for it: check oil before each day’s use, exercise it 2 hours a month at about half load if you use it rarely, and add fresh fuel and stabilizer before storing it more than 120 days.
How do I reset power in my RV when the generator is running?
Turn off big loads like the air conditioner. Reset the breaker on the generator, then the main breaker in the RV panel. Add loads back one at a time; if a breaker trips the moment one appliance comes on, Cummins says that appliance or circuit probably has a short.