Fault Code 420
Cummins
Generator Undervoltage Protection Trip
Generator shuts down; controller displays Fault Code 420 – Undervoltage
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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
Weak excitation due to failing AVR, Loose generator stator or rotor connections, Excessive load causing voltage sag, Worn brushes or slip rings (if brush-type)
How to Fix / Troubleshooting
Safety first: Disconnect the generator from the load and disable automatic start.
- Check load: Ensure the connected load does not exceed the generator’s kW and kVA rating. Reduce load and retest.
- Inspect connections: Tighten main output lugs and internal stator/rotor connections accessible in the terminal box.
- AVR and excitation: Inspect the AVR wiring and excitation leads. If voltage is low even at no load, the AVR or exciter circuit may be failing and require replacement.
- Brushes/slip rings: On brush-type alternators, inspect brushes for wear and ensure they move freely in their holders. Clean slip rings if heavily oxidized.
- Reset: After corrections, clear the fault and verify voltage stability under gradual load.
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Repair Difficulty
Hard
4/5
Required Part
Automatic Voltage Regulator (AVR)
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