AC Output Overvoltage Shutdown Detected
What does ONAN-12 mean?
The ONAN-12 (Cummins Onan) diesel fault code means: AC Output Overvoltage Shutdown Detected. This is a serious severity code.
Common Symptoms
- Generator starts normally then shuts down within seconds to minutes of reaching full speed
- Panel LED flashes 12 times in a repeating pattern after shutdown
- Lights or appliances in the RV flicker or burn unusually bright just before shutdown
- Generator will not stay running and immediately faults when restarted
- STOP light illuminates on the panel after the unit trips off
- Some sensitive electronics such as TVs or inverters may show protection errors after the event
- Generator runs fine at no load but trips when a large load is suddenly removed
Probable Causes (Ranked by Likelihood)
- Faulty or failing AVR (automatic voltage regulator) allowing voltage to climb uncontrolled Very Likely
- Sudden large load drop, such as air conditioner compressor cycling off, causing a momentary voltage spike the AVR cannot damp fast enough Very Likely
- Excitation capacitor out of spec or shorted, pushing field voltage too high Likely
- Loose or corroded control board sensing wire giving the controller a false low-voltage reading, causing the AVR to over-excite the field Likely
- Brushless exciter rotor or stator winding partially shorted, producing excessive field current Possible
- Voltage sensing resistor on the control board drifted out of tolerance Possible
- Main alternator stator winding fault causing unbalanced output and a voltage spike on one leg Less Likely
Step-by-Step Diagnostic Procedure
Before doing anything, turn the panel switch to OFF, disconnect shore power if present, and let the unit cool for 10 minutes. Safety first.
Check AC output voltage at the generator's outlet or at the shore power connection using a multimeter set to AC volts. Normal output for most Onan RV units is 120 V AC plus or minus 5 percent (114-126 V). If you can capture a reading during a brief restart attempt before it trips, anything consistently above 130 V confirms overvoltage.
Inspect all wiring harness connectors leading to the control board. Look specifically for the two-wire voltage sensing lead that runs from the stator output back to the board. A loose pin or green corrosion on that connector is a common cause of a false overvoltage command.
Locate the excitation or run capacitor (a round or oval metal can, usually mounted near the control board or on the stator end bracket). Inspect for bulging, leaking oil, or burn marks. A capacitor that tests out of its rated microfarad value, which you can check with a capacitor-capable multimeter, should be replaced before further testing.
If you have a multimeter with an AC voltage range, measure the voltage across the capacitor terminals while the unit is running and trips. Rated voltage is printed on the capacitor. Values far above or below the label spec confirm a capacitor fault.
Inspect the AVR module on the control board. On many Onan DCB units the AVR is a separate plug-in card or a small potentiometer-equipped board. Look for scorched components, cracked solder joints, or burn marks. If the unit has an external AVR trim pot, do not adjust it unless you have a known-good voltage reference and a service manual. Incorrect adjustment will worsen the fault.
If the above checks are clean and the unit still trips code 12 immediately on restart, the fault is most likely internal to the control board or alternator. At this point, use Onan's InPower diagnostic software or bring in a certified Onan technician. Internal stator and AVR board repair requires specialty tools and is not a DIY task.
After any repair, clear the fault by turning the panel switch OFF for 30 seconds, then restart. Monitor AC output voltage at the outlet for at least 5 minutes under a representative load to confirm voltage stays within the 114-126 V range.
Frequently Asked Questions
What does Onan code 12 mean?
Code 12 means the generator measured AC output voltage above its upper safety limit, typically above 130 V on a 120 V unit. The controller shuts the unit down to protect your appliances and the generator itself. The most common reasons are a failing AVR, a bad excitation capacitor, or a sudden loss of load that caused a brief voltage spike.
Can my generator still run with this code?
No. Code 12 is a shutdown fault. The generator will start, detect the overvoltage condition, and shut itself off. It will not produce usable power until the fault is corrected. Repeatedly forcing restarts while the fault is active risks damaging connected appliances and the alternator windings.
How much does it cost to fix?
If the cause is a faulty excitation capacitor, DIY repair runs about $15-$50 in parts and is straightforward for a handy owner. An AVR module swap runs $60-$180 DIY. If the control board or stator winding is the root cause, expect a shop bill of $300-$600 or more including labor and parts.
Will the generator start the next time the power goes out?
It will attempt to start, but if the root cause has not been repaired it will trip code 12 again within seconds of reaching run speed. You should not rely on this generator for emergency backup power until the overvoltage fault is diagnosed and corrected.