Ruud air conditioner indoor unit shows F4; system may shut down or run with poor dehumidification.

Safety first: Turn off power to the indoor unit before removing return grille or access panels.

• Step 1 – Find return sensor: The return air sensor may be mounted in the return duct, on the filter rack, or inside the air handler cabinet.
• Step 2 – Verify placement: Ensure the sensor is exposed to return air, not touching hot components or metal surfaces that could skew readings.
• Step 3 – Inspect wiring and connections: Check for loose terminals at the control board and any visible damage along the wire path. Repair as needed.
• Step 4 – Measure resistance: Disconnect the sensor and test with a multimeter. Compare to Ruud’s chart. Replace if defective.
• Step 5 – Control board check: If F4 persists with a known-good sensor, the indoor board may need replacement by a technician.