Heater locks out with F03 after several failed ignition attempts; blower may run, but no sustained flame.

Safety first: Turn off power and gas before servicing. Flame loss can indicate combustion or venting problems that may lead to carbon monoxide hazards.

• Check venting: Inspect the exhaust vent and intake (if direct-vent) for obstructions such as leaves, nests, or snow. Ensure vent terminations meet clearance requirements and are not blocked.
• Clean flame sensor: Remove the flame sensor rod and gently clean it with fine emery cloth to remove oxidation. Reinstall securely and ensure the sensor tip is properly positioned in the burner flame path.
• Observe operation: With panels reinstalled, restore power and gas. Call for heat and watch through the sight glass. If the flame lights but drops out after a few seconds, suspect a weak flame signal or gas pressure issue.
• Check gas supply stability: If other gas appliances dim or fluctuate when the heater runs, there may be a supply or regulator issue. Do not attempt to adjust gas regulators yourself; contact a licensed gas technician.
• Professional diagnosis: If F03 persists after cleaning the sensor and confirming clear venting, the gas valve or control board may be faulty and should be tested and replaced by a qualified technician.