Gas Log Pilot Lights but Main Burner Does Not Decision Tree

Why this matters

A vented gas log set whose pilot lights and stays lit but whose main burner will not fire on demand is a tightly bounded fault tree. The candidates are the thermopile output, the main valve, the wall switch or remote, the safety interlock, and the orifice or main burner itself. Walking the tree at the appliance with a multimeter saves a return trip for parts and protects against the common error of replacing the whole valve when the thermopile alone needs replacement. NFPA 54 and the manufacturer's installation manual together define the operating parameters; the multimeter readings against those parameters are the diagnosis.

Symptom presentation

Confirm the failure mode. Pilot lights from cold and stays lit indefinitely; the thermocouple side of the safety circuit is producing enough millivolts to hold the pilot solenoid. The main burner does not come on when the wall switch is closed; the main valve is not opening. The failure sits between the pilot-proven state and the main valve actuation.

Pull the appliance manufacturer and model. Gas log sets from major manufacturers (Real Fyre, Empire, RH Peterson, Monessen) each use slightly different valve and control configurations; the safety interlock topology matters for the diagnostic order. Wall-switch and millivolt-control models behave differently from 24 VAC controlled models with a remote receiver.

Quick checks at the valve

Pull a multimeter and measure thermopile output at the TH-TP-TH-TR terminals of the gas valve with the pilot lit. A healthy thermopile produces 350 to 750 millivolts open-circuit and 100 to 200 millivolts under load. Under 100 millivolts under load indicates a tired thermopile that may still hold the pilot but cannot actuate the main valve operator. Replace the thermopile.

Confirm the wall switch or remote is sending the call. Close the switch and verify continuity across the switch terminals with the valve disconnected and the pilot off. An intermittent switch produces an intermittent symptom; replace with the OEM part. For 24 VAC controlled models, verify the receiver is powered, the batteries are good, and the receiver is sending the call signal to the valve per the OEM service procedure.

Isolation tree

Start with the thermopile reading.

Thermopile under 100 millivolts under load: replace the thermopile. The pilot may continue to hold but the main valve operator cannot pull in against the spring at low millivolts. After replacement, verify the new thermopile reads 350+ millivolts open-circuit and over 200 millivolts under load before closing the access cover.

Thermopile good: check the wall switch or remote circuit.

Switch closed but no main: read voltage at the valve's switch terminals with the switch closed. A healthy switch closes the circuit and the valve sees the thermopile millivolts across its operator coil. No voltage at the switch terminals with switch closed indicates a wire break or a faulty switch; trace the wire run, repair, and retest.

Switch closed, voltage at switch terminals correct, main still does not open: the main valve operator is faulty. Replace the gas valve per the OEM service procedure. Verify the replacement valve matches the gas type and pressure rating exactly; natural gas and propane valves are not interchangeable.

Safety interlock open: vented gas log sets installed in fireplaces typically include a damper-blocking device, a flue blockage sensor, or both. Confirm the damper is open and any flue-blockage sensor or oxygen depletion sensor is in the proven state. ANSI Z21.11.2 covers the oxygen depletion sensor requirements for unvented sets; vented sets per NFPA 54 require the damper to be locked open or the chimney damper interlock to be in the proven open state.

Confirming the call

Document the thermopile reading open-circuit and under load, the switch or remote signal status, the valve voltage at switch terminals, the damper or flue interlock status, and the gas type confirmation on the valve. Schedule a confirmation cycle after the fix; pilot light, wall switch closed, main fires within the OEM-specified ignition window, and stays lit through a full burn cycle.

Remediation by branch

Thermopile failed or marginal: replace with OEM part matched to the valve and the appliance. Verify 350+ millivolts open-circuit before reinstall.

Wall switch failed: replace with the OEM part. Some sets use a millivolt-rated switch that cannot be substituted with a standard switch; verify the rating.

Remote receiver or batteries: replace batteries first as the cheap test. If the receiver is not signaling, replace the receiver per the OEM service procedure. Pair to the remote per the OEM steps.

Wire run damaged: repair the wire and retest. Document the run path in the customer record for future service.

Gas valve operator failed: replace the valve. Verify gas type, pressure rating, and electrical rating before installation. Pressure-test all joints with a soap solution or electronic leak detector per NFPA 54 leak test procedures.

Damper or flue interlock open: confirm damper is locked open per NFPA 54 vented gas log set installation requirements. If a flue blockage sensor is installed, verify the sensor is proving correctly per the OEM service procedure.

References

1. NFPA 54, National Fuel Gas Code, 2021 edition.
2. NFPA 211, Standard for Chimneys, Fireplaces, Vents, and Solid Fuel-Burning Appliances, 2024 edition.
3. ANSI Z21.50, Standard for Vented Gas Fireplaces, current revision.
4. ANSI Z21.11.2, Standard for Gas-Fired Room Heaters, Volume II, Unvented Room Heaters, current revision.
5. CSIA, Certified Chimney Sweep Reference Manual, current edition.