Onan Generator: “Fault 15”
warning RV generator fault codes
What it means
Underfrequency fault. The engine governor could not keep the engine spinning fast enough to maintain rated output frequency (Hz), so the controller shut the unit down.
Most likely causes
- Excessive electrical load overworking the engine/governor (A/C plus chargers) most likely
- On high-pressure LPG models: oil migrated from the supply system into the genset LPG plumbing, starving fuel common
- Air cleaner restriction or a fuel-delivery problem limiting power less common
Safe checks you can do yourself
- Reduce the number of connected appliances, especially air conditioners and active battery chargers, then restart.
- On high-pressure LPG models only: with the system depressurized, drain accumulated oil from the genset LPG drain valve per the manual, then be sure to re-close the drain valve.
- Check that the air cleaner is not clogged.
How to reset / clear Fault 15
- Shed load and/or clear the LPG oil, then restart from the control switch.
- Confirm the unit holds frequency under your normal load.
When to call a technician
If underfrequency continues after reducing load and (on LPG units) draining oil — the governor or fuel system likely needs a dealer.
Accuracy notes
Quiet Diesel definition: frequency below 54 Hz for more than eight seconds during normal operation. As with code 13, the manufacturer's first corrective action is fewer connected loads — especially motor starting loads.
Applies to
Marquis Gold · QG 4000 · QG 5500 · KY series · HGJAA · HGJAB · HGJAC · HGJAD · HGJAE · HGJAF
FAQ
What does Onan Generator code "Fault 15" mean?
Underfrequency fault. The engine governor could not keep the engine spinning fast enough to maintain rated output frequency (Hz), so the controller shut the unit down.
How do I reset Onan Generator Fault 15?
Shed load and/or clear the LPG oil, then restart from the control switch. Confirm the unit holds frequency under your normal load.
What is the most common cause of Fault 15 on a Onan Generator?
The most likely cause is excessive electrical load overworking the engine/governor (a/c plus chargers).
Related · Onan generator
Undervoltage fault. The controller could not hold the generator's AC… Fault 22
Governor actuator overload. The controller had to hold the governor… Fault 14
Overfrequency fault. The engine governor could not hold the rated output…
Sources
- Onan Troubleshooting Codes (Service Manual Table 5) — Underfrequency Fault — Code No. 15 (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