Is there a call for heat?

Set the thermostat above room temperature. Verify 24 VAC between R and W at the furnace control board. The IFC board should receive the call.

Are all limit switches closed?

Verify continuity through the high-temperature limit switch, rollout switch(es), and any auxiliary limits in the 24 V safety circuit. All must be closed for the IFC board to proceed.

Does the inducer draft motor start?

After the call for heat is confirmed and limits are satisfied, the IFC board should energize the inducer motor. Listen and look for the inducer wheel spinning. It may take a few seconds.

Does the pressure switch close?

Once the inducer is running and draft is established, the pressure switch should close. You can verify with a meter across the pressure switch terminals — closed = near 0 Ω.

Is spark present at the igniter?

With the pressure switch proven closed, the IFC should energize the spark igniter. Look and listen for spark at the burner area. Spark should occur within a few seconds of the pressure switch closing.

Does the main burner light?

The IFC board should open the gas valve simultaneously with (or shortly after) spark. Watch for main burner ignition. If gas is flowing and spark is present, the burner should light within seconds.

Does flame stay established?

After the burner lights, the IFC board must detect flame via the flame sensor within a few seconds. If flame is not sensed, the board will shut down the gas valve and may retry or lock out. Most boards allow 3 retries before lockout.

Does the blower motor start?

After a timed delay (typically 30–60 seconds), the IFC board should energize the blower motor to distribute heated air. The blower should ramp up to the appropriate heating speed.

Does the system run normally until heat is satisfied?

The furnace should continue running with burner lit and blower operating until the thermostat is satisfied. Check the temperature rise (Delta T) across the heat exchanger — it should fall within the range on the furnace rating plate (typically 30–60°F). If the system short-cycles (shuts off before the thermostat is satisfied), a limit switch may be tripping.

Does the gas valve close when thermostat is satisfied?

When the thermostat reaches setpoint and the call for heat ends (R-W opens), the IFC board should de-energize the gas valve. The flame should extinguish immediately.

Does the inducer motor stop after post-purge delay?

After the gas valve closes, the inducer motor should continue running for a post-purge period (typically 15–60 seconds depending on model) to clear combustion gases, then shut off.

Does the blower stop after off-delay?

After the burner shuts off, the blower should continue running for an off-delay period (typically 60–180 seconds) to extract residual heat from the heat exchanger, then stop. This delay is usually set by a dip switch or jumper on the IFC board.

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Gas Furnace — IFC / Spark Ignition

IFC / Spark Ignition Furnace Troubleshooting

Step-by-step diagnostic workflow for gas furnaces with an Integrated Furnace Control (IFC) board and spark ignition. Covers call for heat through normal operation, shutdown sequence, and likely fault identification.

Safety Notice

Disconnect all power before accessing electrical components. Verify zero voltage with a reliable meter. Natural gas is present — if you smell gas, leave the area and call your gas utility. Follow all LOTO (Lock-Out / Tag-Out) procedures. Wear appropriate PPE including safety glasses and insulated gloves when taking live measurements.

Step 1

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Before You Begin

This workflow guides you through the standard operating sequence of an IFC spark-ignition gas furnace. You will verify each stage and identify where the sequence fails. Have your multimeter, manometer, and flashlight ready before starting.

Related Calculators

Voltage Drop→Single Phase Power→Delta T→

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Digital Multimeters→Combustion Analyzers→

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IFC / Spark Ignition Furnace Troubleshooting

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