Code 42
Onan
Loss of Excitation / Field Circuit Fault
Generator shuts down; Code 42 displayed indicating loss of field or excitation
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Safety Warning
This repair may involve working with high voltage components or water connections. Always unplug the appliance before removing any panels.
If you are not confident in your ability to perform this repair safely, we strongly recommend contacting a professional technician.
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Possible Causes
Open field winding in rotor, Failed voltage regulator, Broken field wiring or slip ring/brush issues, Defective controller excitation driver
How to Fix / Troubleshooting
Safety first: High voltage and rotating parts are present. Open the main breaker, disable auto-start, disconnect battery negative, and only proceed if you are qualified.
- Inspect brushes and slip rings: On brush-type alternators, check brushes for wear and ensure they move freely in their holders. Clean slip rings and replace worn brushes.
- Check field wiring: Inspect field leads from regulator to rotor for continuity and secure connections.
- Test rotor field resistance: Measure rotor field resistance per Onan specifications. An open circuit indicates a failed rotor.
- Voltage regulator: If rotor and wiring are good, test and replace the voltage regulator as needed.
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Repair Difficulty
Professional Required
5/5
Required Part
Alternator Rotor Field
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