When a furnace stops working, follow this checklist before ordering parts. Experienced techs run through these steps in order to avoid unnecessary part replacements.
Step 1: Check the Thermostat
Before anything else, verify the thermostat is set to HEAT and the set temperature is above room temperature. Check batteries if it is a wireless model. Try setting it 5 degrees above room temp and wait 2 minutes.
Step 2: Check the Power
Verify the furnace switch (looks like a light switch, usually on the side of the furnace or nearby wall) is ON. Check the breaker. Check for a blown fuse on the control board (small 3A or 5A automotive-style fuse).
Step 3: Check the Filter
A severely clogged filter can cause the furnace to overheat and shut down on the high-limit switch. Pull the filter and try running without it briefly to test.
Step 4: Check for Flashing Error Codes
Most modern furnaces have an LED on the control board that flashes a diagnostic code. Count the flashes and check the code chart on the inside of the furnace door panel.
Step 5: Check the Flame Sensor
If the furnace tries to ignite but shuts down after a few seconds, the flame sensor is likely dirty or failed. Remove it and clean with fine steel wool or emery cloth. If cleaning does not help, replace it. Cost: $8-20.
Step 6: Check the Igniter
If the furnace does not attempt to ignite at all (no glow from the hot surface igniter), the igniter has likely cracked. They are fragile ceramic components. Do not touch the element with bare hands. Cost: $15-40.
Step 7: Check the Inducer Motor
The inducer motor (draft motor) must run before ignition starts. If you do not hear the inducer start when the thermostat calls for heat, the inducer motor or its capacitor may have failed. Cost: $80-200.
Step 8: Check the Pressure Switch
If the inducer runs but the furnace does not progress to ignition, the pressure switch may not be closing. This can be caused by a blocked drain, cracked pressure hose, or a failed switch. Cost: $15-35.
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