Ruud air conditioner stops cooling and displays E4 or flashes a fault code; unit may run briefly then shut down.

Safety first: Turn off power to the indoor air handler/furnace before removing panels. Avoid bending refrigerant lines or damaging the coil.

• Step 1 – Replace or clean air filter: Locate the return air filter in the grille or air handler. Replace disposable filters or clean washable ones. Restricted airflow is the most common cause of coil freeze.
• Step 2 – Verify blower operation: Set thermostat to FAN ON. Confirm the Ruud indoor blower runs smoothly and at normal speed. If it does not run, check the blower motor, capacitor, and control board.
• Step 3 – Thaw the evaporator coil: Turn the system OFF (cooling) and set thermostat to FAN ON only. Allow 2–4 hours for ice to melt. Place towels or a pan to catch excess condensate.
• Step 4 – Inspect evaporator coil and sensor: After thawing, inspect the coil for heavy dirt buildup and clean with a non-rinse coil cleaner. Locate the evaporator thermistor clipped to the coil tubing; ensure it is firmly attached and not dangling.
• Step 5 – Call a technician if freezing recurs: Persistent E4 after airflow corrections may indicate low refrigerant or a defective thermistor. A technician should check superheat/subcooling and replace the sensor if out of spec.