Generator runs but controller shows 'Undervoltage' and may not transfer load; AC output is significantly below nominal (e.g., 180–200 VAC on a 240 V system).

Safety: Isolate the generator from the building by opening the generator breaker and transfer switch. Use insulated tools and a CAT III/IV rated meter.

• Check engine performance: Listen for engine bogging or surging. Verify fuel supply and pressure for NG/LP. Clean or replace the air filter and inspect spark plugs. A weak engine cannot maintain proper RPM and voltage.
• Inspect AVR and wiring: Locate the AVR and check for burnt components or discoloration. Verify tight connections on the stator sensing leads and output leads. Replace the AVR if visibly damaged or if voltage does not respond to adjustment (if adjustable).
• Check stator resistance: With the engine off and battery disconnected, measure resistance between stator windings and to ground. Very low resistance or continuity to ground indicates a shorted stator. Replace the stator assembly if defective.
• Retest: After repairs, run the generator with no load and verify correct voltage, then apply a moderate load and confirm stability.