Furnace displays F09; unit may short-cycle or fail to reach set temperature.

Safety Warning

This repair may involve working with high voltage components or water connections. Always unplug the appliance before removing any panels.

If you are not confident in your ability to perform this repair safely, we strongly recommend contacting a professional technician.

Possible Causes

Return air sensor drifted out of spec, Sensor installed in wrong location (too close to heat source), Loose sensor connection at IFC, Defective IFC sensor circuit

How to Fix / Troubleshooting

Safety: Turn off power before handling sensors.

Steps:

Locate the return air sensor in the return duct or blower compartment.
Ensure it is not touching metal surfaces that get hot or located where it senses supply air.
Check the wiring and connector for tightness and damage.
Measure sensor resistance and compare to the Westinghouse temperature/resistance chart. Replace if out of range.
If a new sensor does not resolve F09, the IFC may require replacement.

Repair Difficulty

Medium 3/5

Required Part

Return Air Temperature Sensor

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