Is there a call for heat?

Verify the thermostat is calling for heat. Check for 24 VAC between R and W at the equipment terminals. If using a communicating system, confirm the call is active on the board diagnostics. No call for heat = thermostat, wiring, or transformer issue — not an ignition problem.

Does the ignition module have power?

Measure voltage at the ignition module terminals. Depending on the system, you should see 24 VAC or 120 VAC when the call for heat is active. Check module documentation for expected input voltage. If the module has no power, trace back through limits (high-limit, rollout, pressure switch) and transformer.

Is there spark at the pilot?

With the call for heat active and the module powered, observe the pilot assembly. You should see a visible spark jumping from the ignition electrode to the ground point near the pilot burner. If you can hear clicking from the module but see no spark at the pilot, the issue may be in the ignition cable, electrode, or ground path rather than the module itself. Darken the room if needed to see weak spark.

Does the pilot burner light?

With spark confirmed, does a pilot flame appear? If the spark is strong and consistent but no pilot flame appears: • Gas supply to pilot may be blocked (pilot orifice, pilot tubing, manual valve) • Air in the pilot gas line (new install or after service) • Pilot orifice clogged with debris or spider webs • Pilot gas supply valve not open If the flame appears briefly then goes out, note whether it goes out during the spark or after spark stops.

Does the spark stop once pilot is proved?

On a properly operating intermittent pilot system, spark should stop once the module detects flame. Continuous sparking after pilot is lit indicates the module is not seeing the flame signal. If spark continues with a good pilot flame: • Flame rod not in the pilot flame • Flame rod or sensing circuit issue • Module flame input not receiving signal • Polarity or grounding problem

Does flame signal improve after service?

Reinstall the cleaned or repositioned flame sensor / thermocouple and run the system. Measure the flame signal again. If the signal is now in the normal range and the system progresses to main burner — the issue was a dirty or mispositioned sensor. If the signal is still weak or absent, the rod may be worn beyond cleaning, or the issue is elsewhere in the circuit.

Does the system run through the full heating cycle?

With the main burner lit, observe the full cycle: • Does the system run until the thermostat is satisfied? • Does the blower come on at the appropriate delay? • Does the system shut down cleanly when the call ends (main valve closes, blower runs on delay, then off)? If the system drops out mid-cycle, note when and how — this indicates an intermittent fault.

What happens when it drops out?

Determine what shuts down first: • Main burner goes out, then system locks out → flame loss during run (flame rod drift, gas pressure fluctuation, or intermittent wiring) • System shuts down on limit → overheating due to airflow restriction, dirty filter, or blower issue • Power interruption — lights on module blink off momentarily → loose connection, intermittent limit, or transformer issue

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Gas Furnace / Boiler with Intermittent Pilot Ignition (spark-to-pilot)

Intermittent Pilot Troubleshooting

Q: Does the main gas valve open?

With pilot proved, the ignition module should energize the main gas valve. You should hear the valve click open and see main burner ignition within a few seconds. If the pilot is proved but the main valve does not open: • Check for 24 VAC at the main valve operator terminals (from the module) • If voltage is present but valve doesn't open — valve or operator may be stuck/failed • If no voltage — module is not sending the signal despite proving flame

Step-by-step diagnostic for intermittent pilot ignition systems — from spark failure through flame proving, gas valve operation, and shutdown behavior.

Safety Notice

Disconnect power before handling wiring. Gas-fired equipment produces lethal voltages, combustion gases, and hot surfaces. Only trained, qualified technicians should perform these procedures. Follow NFPA 54 and local codes. Never bypass safety controls.

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Intermittent Pilot Troubleshooting Overview

This workflow covers intermittent pilot ignition systems where a spark ignites a small pilot flame, the flame is proved via a flame rod or thermocouple, and only then does the main gas valve open. Common symptoms: • No spark at pilot • Spark present but pilot won't light • Pilot lights but flame isn't proved • Main burner fails to open • System locks out after retries Before starting: • Confirm the thermostat is calling for heat • Confirm gas supply is on • Note any error codes on the ignition module or IFC board

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Intermittent Pilot Troubleshooting

Intermittent Pilot Troubleshooting Overview

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