Onan and Cummins generators (Onan is a Cummins brand) are the most common RV generators on the market. They're reliable, but they have well-documented failure patterns that account for the vast majority of no-start calls. Work through this checklist in order before scheduling service.
Step 1: Fuel — the most common cause
Onan generators pull fuel from the same tank as the motorhome engine, but they draw from a pickup located at a different point in the tank. This means the generator can run out of "its" fuel while the engine gauge shows 1/4 tank remaining. Onan specifies that the generator should not be used when the tank is below 1/4 full. If you're below 1/4, add fuel first.
If the tank is adequately full, the next fuel issue to check is the fuel solenoid. The Onan has a fuel solenoid that shuts off fuel supply when the generator isn't running. When these solenoids stick closed (usually after a long period of inactivity), the generator will crank but not start. You'll hear the starter motor engage, but the engine won't fire. Tapping the solenoid housing with a screwdriver handle while cranking can sometimes free a stuck solenoid temporarily. The solenoid is replaceable — it's a common and inexpensive repair.
Step 2: Oil level and oil pressure shutdown
Onan generators have a low-oil pressure shutdown that will prevent the generator from starting if the oil level is low or if the oil pressure sensor detects a problem. Check the oil level on the dipstick with the generator cold and level. If it's low, top it off with the correct weight (usually 15W-40 in warm climates) and try starting again.
If the oil level is correct but the generator still won't start, the oil pressure sensor itself may have failed — a false reading of low pressure will prevent startup. This is a straightforward diagnostic for a tech with the right equipment. Don't bypass oil pressure safeties.
Step 3: The carburetor — especially after sitting
If your generator has been sitting for more than 30–60 days without running, the carburetor is the most likely culprit. Gasoline goes stale and leaves varnish deposits in the carburetor's small orifices, which block fuel flow. The generator will crank, may briefly fire, but won't sustain operation.
For a generator that starts then dies, or that won't stay running: with the generator running (or trying to run), spray a short burst of carburetor cleaner into the air intake. If the engine momentarily catches or runs better, you've confirmed the carburetor is starved for fuel. The fix is carburetor cleaning or replacement.
The Onan carburetor is accessible without removing the generator from the coach on most installations. The main jet is the primary item to clean. Many owners do this themselves; if you're not comfortable, it's a 1–2 hour service call for an experienced tech.
Step 4: Spark plug condition
Onan generators use one or two spark plugs depending on the model. Plugs foul with carbon after extended use or after a period of running at low load (which is common in RV use — running the generator just to charge batteries or run a small load at low RPM accelerates fouling). Remove the plugs and inspect them. Heavy carbon deposits, oil fouling, or a gap that's widened from wear all indicate plugs that need replacement. Plugs are a $5–10 part and are worth replacing at first sign of trouble.
Step 5: Battery and starter circuit
The generator has its own starter circuit, typically drawing from the chassis battery (not the house batteries). If the chassis battery is discharged or the starter connections are corroded, the generator may not crank at all — or will crank slowly and not generate enough compression to fire. Verify the chassis battery voltage (should be above 12.4V) and that the terminals and connections at the starter solenoid are clean and tight.
Step 6: Load testing and the hour meter
If the generator starts but immediately shuts down, check whether it's shutting down on an overload. Running an air conditioner, microwave, and electric water heater simultaneously can exceed the generator's rated output, triggering the circuit breaker. Start with minimal load and add appliances one at a time to isolate an overload situation.
Check your generator's hour meter. Onan specifies service intervals at 100, 150, or 200 hours depending on the model. Oil changes, filter replacements, and spark plug replacement at these intervals prevent the majority of field failures. If your hour meter is significantly past the last service, that context matters.
When to call a tech
If you've worked through fuel, oil, the carburetor, plugs, and the battery — and the generator still won't start or won't stay running — the problem is likely electrical: a failed control board, a fault code stored in the generator's controller, a failed voltage regulator, or an internal engine issue. At this point, a tech with an Onan diagnostic reader can pull the fault codes in minutes and tell you exactly what the generator's own diagnostic system recorded as the failure point.
Search the SMART-RV case database by your Onan model number (found on the generator's data plate) and symptom. Cases from RVTI-credentialed technicians include the fault code, root cause, parts replaced, and repair time — giving you a realistic picture of what you're dealing with before you make the call.