Onan Generator: “Fault 12”
warning RV generator fault codes
What it means
Overvoltage fault. The controller could not keep the output voltage at the rated value (output voltage too high) and shut the genset down. Read as one blink, a pause, then two blinks.
Most likely causes
- Controller / voltage regulation fault common
- Voltage-sensing circuit problem less common
Safe checks you can do yourself
- Confirm the blink pattern is 1 then 2 (overvoltage 12), not 1 then 3 (undervoltage 13).
- Avoid running sensitive electronics until resolved, since voltage may be out of spec.
How to reset / clear Fault 12
- Official corrective action: see an authorized Onan dealer.
- Re-read the logged code with three Stop presses within 5 seconds (code stops blinking after ~5 minutes).
When to call a technician
The manual directs code 12 to an authorized Onan dealer - overvoltage is not a homeowner repair.
Accuracy notes
The Quiet Diesel manual gives the trip points the air-cooled manuals leave out: output above 150 VAC, 125 % of rated, for 75 milliseconds, or above 138 VAC, 115 % of rated, for three seconds. Both manuals send this one to a dealer.
Applies to
Onan RV QG / KY series RV gensets · Onan blink-code-controller RV gensets
FAQ
What does Onan Generator code "Fault 12" mean?
Overvoltage fault. The controller could not keep the output voltage at the rated value (output voltage too high) and shut the genset down. Read as one blink, a pause, then two blinks.
How do I reset Onan Generator Fault 12?
Official corrective action: see an authorized Onan dealer. Re-read the logged code with three Stop presses within 5 seconds (code stops blinking after ~5 minutes).
What is the most common cause of Fault 12 on a Onan Generator?
The most likely cause is controller / voltage regulation fault.
Related · Onan generator
Undervoltage fault. The controller could not hold the generator's AC… Fault 27
Voltage sense fault. The controller was unable to sense the generator's… Fault 29
High battery voltage fault. The voltage across the genset's starting…
Sources
- Onan RV Generator Troubleshooting Codes (Table 5) — TABLE 5 - OVERVOLTAGE FAULT-CODE NO. 12 (Controller unable to maintain rated voltage) (Cummins/Onan (hosted by Heartland Owners))
- Operator Manual — RV Generator Set KY (Spec J–N) — 981-0159 (Issue 9), Section 4 Troubleshooting, Table 2 (Cummins Inc. (Onan))
- Operator Manual — RV Generator Set HDKCA / HDKCB (Spec A–E) — 0981-0173 (Issue 10), Section 4 Troubleshooting, Table 4-1 (Cummins Inc. (Onan Quiet Diesel))
● Checked against 3 manufacturer documents · Last verified 2026-08-07 · compiled from manufacturer documentation. Codes can vary by model year — always cross-check your unit’s manual. How we verify