Generator will not start and the controller display shows E01 immediately after a start attempt.

Safety first: Turn the generator to OFF, disconnect the battery negative cable, and ensure the unit cannot auto-start (set to OFF/Manual, not AUTO). Allow the engine to cool completely.

• Check oil level: Remove the dipstick and verify oil level is between the LOW and FULL marks. Top up with the oil grade specified in the Firman standby generator manual (commonly SAE 10W-30 or as specified) and recheck.
• Inspect for oil leaks: Look around the oil filter, drain plug, crankcase gasket, and oil cooler (if equipped) for wet or oily areas. Repair leaks before restarting.
• Test the oil pressure switch: Locate the oil pressure switch on the engine block (usually near the oil filter). Disconnect its wire and check continuity with a multimeter. With engine off, most low-oil switches are closed to ground; if it reads open or shorted incorrectly, it may be faulty. Temporarily disconnecting the switch wire and attempting a start (for test only) can confirm if the switch is falsely triggering. Do not run the engine for more than a few seconds with the switch bypassed.
• Inspect wiring: Trace the harness from the oil pressure switch to the control board. Look for chafed insulation, loose connectors, corrosion, or rodent damage. Clean and reseat connectors.
• Reset and test: Reconnect everything, restore battery, clear the code per the Firman controller instructions (usually by cycling OFF → AUTO or using the reset button), then attempt a start.

If E01 persists with correct oil level and a known-good switch, have a technician check actual oil pressure with a mechanical gauge and inspect the oil pump and internal engine condition.