Onan RV generator fault codes: how to read and use them

Many Cummins Onan RV generators use a start/stop switch light or display to report faults. The exact code retrieval sequence and code meaning depend on the model and spec. A QG 4000, QG 5500, QD diesel and newer inverter model may not use the same procedure or troubleshooting tree.

Find the model/spec before trusting any code list

The data tag is the anchor. Write down the model number, spec letter, serial number, fuel type and hours. Cummins explains that the spec letter identifies the particular generator version, which is why two generators with the same common name can need different manuals.

First-level codes versus detailed codes

Some Onan systems show a first-level blink pattern that tells you a shutdown is stored, then require another step to retrieve the two-digit detail code. If the switch flashes three times, do not stop there; use the manual for the procedure that reveals the actual stored fault.

What a code can and cannot tell you

• A code can point toward low oil pressure, overcrank, overspeed, undervoltage, overvoltage, overheating, stopped engine, control fault or other protected conditions.
• A code cannot prove the failed part by itself. Low oil pressure may be low oil, wrong reading method, sensor wiring, actual pressure trouble or shutdown during a different event.
• A code captured after repeated restarts may describe the last failure, not the first cause.

Write this down before clearing the code

• Outside temperature and altitude if relevant.
• Fuel level and fuel type.
• Oil level and whether the RV was level.
• Loads running at shutdown.
• Whether the generator was cold, hot, under load or just starting.
• Whether the RV was on shore power, inverter or generator-only power.

Related Onan code pages

• Onan generator code 36
• Onan generator 3 flashes
• Onan generator low oil shutdown
• Onan generator starts then dies
• Onan generator runs but no RV power

When a code needs service

Get qualified generator service for repeated shutdown, overspeed, internal voltage output faults, oil-pressure faults that return with correct oil level, overheating with clean airflow, exhaust leaks, fuel leaks, EFI codes, controller faults or any code that requires live testing inside the generator.