Heater displays F07 and will not fire; may occur immediately on power-up.

Safety first: Turn off power and gas. High flue temperatures can indicate dangerous combustion issues.

• Locate stack sensor: Find the Nirvana flue gas temperature sensor mounted on or near the exhaust collector or flue outlet.
• Inspect wiring: Check the sensor leads for heat damage or brittle insulation. Ensure the connector at the control board is tight and clean.
• Check for vent blockage: Inspect the exhaust vent for obstructions or crushed sections that could cause overheating.
• Resistance test: With the heater cool, measure the sensor resistance and compare to the Nirvana spec chart. Replace the sensor if readings are out of range or intermittent.
• Reassemble and test: After replacing the sensor or clearing vent issues, restore power and gas, then call for heat. If F07 persists, the control board may be misreading and should be evaluated by a technician.