Firman standby generator runs but the controller shows E05 and AC voltage is unstable or low.

Safety first: Turn OFF the generator, disconnect battery negative, and lock out the ATS if possible. Wait for all rotating parts to stop.

• Inspect alternator connections: Remove the alternator access cover on the Firman generator. Check all terminal connections from the stator to the AVR and from the AVR to the output lugs. Tighten any loose terminals.
• Check AVR harness: Locate the AVR module mounted near the alternator. Inspect its plug for corrosion, heat damage, or loose pins. Reseat the connector firmly.
• Visual inspection of brushes/slip rings (if equipped): If your Firman model uses brushes, remove the brush holder and inspect for excessive wear or oil contamination. Replace brushes if they are below the minimum length specified.
• Test no-load voltage: With a qualified person and proper meter, run the generator with no load and measure line-to-line voltage. If voltage is very low or fluctuating and wiring is intact, the AVR is a likely failure point.

Replacing the AVR typically restores proper voltage. If a new AVR does not correct E05, a technician should perform insulation resistance and winding tests on the stator and rotor.