Ruud air conditioner indoor unit displays F3; blower may run but cooling is disabled.

Safety first: Turn off power to the indoor unit before accessing ductwork or control boards.

• Step 1 – Locate supply sensor: The supply air sensor is usually mounted in the supply plenum or just downstream of the evaporator coil, wired back to the Ruud control board.
• Step 2 – Check mounting: Ensure the sensor probe is properly inserted into the airstream and not hanging loose. Re-secure with a grommet or clip if needed.
• Step 3 – Inspect wiring: Look for loose connections at the board and any damage along the wire run. Repair or replace damaged sections.
• Step 4 – Test sensor: Disconnect and measure resistance, comparing to Ruud’s temperature/resistance chart. Replace if readings are out of range.
• Step 5 – Board replacement: If a new sensor does not clear F3, the indoor control board may have a failed sensor input and require replacement.