Cummins Onan RV generator fault codes
On many Onan RV sets the status light blinks the first digit, pauses, then blinks the second digit. Write the full number down BEFORE you cycle power. Confirm the model and spec letter because meanings can vary.
Onan generator code 2
Low oil pressure. Check the oil level with the set level. Add if low, drain if over full. QG 4000: about 1.8 qt. SAE 30 above 32°F, 15W-40 for 10 to 100°F, 5W-30 for cold.
Onan generator code 3
Service check: a two-digit code is stored. Press STOP/PRIME once more and count the two groups of blinks (tens, pause, ones).
Onan generator code 4
Over crank: 30 seconds without starting. Fuel above a quarter tank (the generator pickup sits higher than the engine's). Prime 30 s on gasoline. Spark plug clean and gapped, air filter clean, fuel filter. On LP check the cylinder and regulator.
Onan generator code 12
Output over voltage. Generator output too high. Disconnect loads, check the voltage regulation and sensing, then service.
Onan generator code 13
Output under voltage. Overloaded, engine running slow, or a weak alternator. Reduce the load, check engine speed, service.
Onan generator code 14
Output over frequency. Engine speed too high. Check the governor and throttle actuator and linkage.
Onan generator code 15
Output under frequency. Engine running slow or overloaded. Fuel, air, load and the governor.
Onan generator code 19
Throttle actuator circuit fault. Check the actuator plug and wiring, and the linkage for binding. Replace the actuator if needed.
Onan generator code 22
Governor actuator overload: near full duty too long. Reduce the load. Replace a dirty air filter. Check the exhaust for a blockage.
Onan generator code 25
Alternator over voltage. Alternator control fault. Service.
Onan generator code 26
Alternator under voltage. Alternator, wiring or load problem. Check load, then service.
Onan generator code 27
Pma sense lost. The permanent magnet alternator signal is lost. Check the plug and wiring, then the alternator.
Onan generator code 29
Battery over voltage. Check the start battery and the charge circuit. Measure the battery while running.
Onan generator code 31
Over speed: above 3400 rpm. Governor fault. Dealer.
Onan generator code 32
Low cranking speed: under 180 rpm for 2 s. Clean and tighten battery cables at both ends. Charge or replace the battery (450 CCA minimum). Use the right oil viscosity for the weather.
Onan generator code 35
Control card failure (EEPROM). Power cycle at the battery once. If it returns, replace the control board.
Onan generator code 36
Engine stopped without a stop command. Check the fuel level, prime for about 30 seconds per the manual, and check the air filter. Stop if you find a fuel leak or damage.
Onan generator code 37
Invalid generator set configuration. Frequency to rpm ratio is wrong. Dealer.
Onan generator code 38
Over current / field overload. Low power factor loads: a sick AC compressor or a motor drawing badly. Reduce the load and clamp the appliances.
Onan generator code 41
Generator rotor: can't sense field or output voltage. Check the connectors between the control and the generator end for corrosion. Then service.
Onan generator code 42
Processor ROM error. Power cycle. If it returns, replace the control board.
Onan generator code 43
Processor RAM error. Power cycle. If it returns, replace the control board.
Onan generator code 45
Speed sense: can't sense quadrature frequency. Connector and wiring from the generator end, then service.
Onan generator code 47
Ignition: control can't sense ignition. Connector and wiring, then service.
Onan generator code 48
Generator field sense. Connector and wiring, then service.
Onan generator code 51
Processor malfunction. Power cycle. If it returns, replace the control board.
Onan generator code 52
Fuel injector open or short (EFI models). Check the injector connector, then the injector.
Onan generator code 54
Mat sender open or short (efi). Connector, then replace the sensor.
Onan generator code 56
Map sender open or short (efi). Connector, then replace the sensor.
Onan generator code 57
Over prime: more than 3 minutes. Check both control switches for something holding them in STOP/PRIME.
Onan generator code 58
Exhaust gas temperature over 650°C (1202°F). A lean condition or restriction. Air filter, exhaust, carburetor mixture.
Onan generator code 81
Egt sensor open. Sensor below 60°C for 2 minutes running or not above 60°C within 2 minutes of start. Replace the sensor.
Onan generator code 82
Egt sensor shorted. Reads 1000°C. Replace the sensor.
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Guided diagnosis
Full field guide: Cummins Onan RV generator
Codes come from the maker manuals and public service sheets. Your model's own chart always wins, and LP and 120 volt work belongs to a qualified technician.