Generator shuts down with F06 Underspeed; engine sounds loaded and slow before trip.

Safety first: Turn the generator to OFF and open the output breaker before working on the unit. Avoid operating with severe underspeed, as it can damage connected loads.

• Reduce load: Check the Siemens automatic transfer switch (ATS) or load center and identify large loads (HVAC, water heaters, EV chargers). Temporarily switch off non-essential loads and retest the generator under reduced load to see if F06 clears.
• Inspect fuel system: For gas units, verify gas pressure and that the regulator vent is clear. For diesel units, replace clogged fuel filters and bleed air from the fuel system using the manual priming pump if equipped.
• Check governor actuator: Observe the governor actuator arm during a start attempt. If it moves sluggishly or fails to open the throttle under load, inspect the actuator linkage and electrical connector. Replace the actuator if it does not respond to control signals.
• Verify speed sensor wiring: Loose or intermittent speed sensor connections can cause false underspeed readings. Reseat the connector at the sensor and controller, and secure the harness away from hot exhaust components.
• Retest frequency: With a multimeter capable of frequency measurement, verify output frequency at the generator terminals (should be near 50 or 60 Hz depending on model). Persistent low frequency under normal load indicates a governor or engine power issue requiring professional service.