Heater will not fire, display shows E01 shortly after a call for heat.

Safety first: Turn off power at the breaker and shut off the gas supply before opening the heater. Allow the unit to cool completely.

• Check gas supply: Verify the gas valve to the heater is fully open and that other gas appliances are working. If gas supply is questionable, contact your gas utility or a licensed gas technician.
• Inspect the hot surface igniter: Remove the front panel, locate the igniter near the burner tray, and check for cracks, heavy white deposits, or broken wires. Clean light soot with a soft brush; do not touch the element with bare fingers. Replace the igniter if cracked or heavily worn.
• Check igniter wiring and connectors: Ensure the harness from the ignition control module to the igniter is fully seated and not heat-damaged or brittle. Repair or replace damaged wiring.
• Verify air flow: Make sure the blower (on forced-draft models) starts and runs smoothly. Clear any debris from the intake and exhaust vents that could affect combustion.
• Reset and test: Restore power and gas, clear the error by cycling power, and call for heat. If E01 returns after verifying gas and igniter, the ignition control module or gas valve may need professional diagnosis and replacement.