Fault 12

Onan Generator: “Fault 12”

warning RV generator fault codes

Safety Generator output is line-voltage AC and can be lethal. Do not open the genset or work on output wiring. This is also a fuel-burning engine - if you smell gas/propane, stop and ventilate; never run enclosed (carbon monoxide).

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.

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.

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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))

Note: Code 13 (undervoltage) has a DIY angle - reduce connected appliances, especially A/C and battery chargers; code 12 (overvoltage) does not and goes to a dealer.

● Last verified 2026-06-29 · compiled from manufacturer documentation. Codes can vary by model year — always cross-check your unit’s manual.