Thermostat or indoor display shows E4 and the system cycles off quickly after starting

Safety first: Turn off power to the indoor unit at the breaker before removing access panels.

• Locate the coil sensor: On American Standard air handlers, the evaporator temperature sensor (thermistor) is usually clipped to the refrigerant tubing entering or exiting the coil or inserted into the coil fins.
• Check sensor attachment: Ensure the sensor is firmly clipped to the tubing or properly inserted into the coil. A loose sensor can misread temperature and trigger E4.
• Inspect wiring: Follow the sensor wires back to the control board. Look for pinched, cut, or disconnected wires. Reseat the connector on the board.
• Clean the coil: A heavily clogged coil can cause freezing and sensor errors. If accessible, gently brush off dust and use a no-rinse evaporator coil cleaner following the product instructions.
• Replace sensor if defective: If the sensor is physically damaged or corroded, replace it with the correct American Standard evaporator thermistor. If E4 remains after replacement, the control board may need professional evaluation.