Keystone window or portable air conditioner displays E1 and stops cooling or controls are unresponsive.

Safety first: Unplug the air conditioner from the wall outlet before removing any covers. Capacitors can hold a charge; avoid touching exposed terminals.

Steps to check and correct:

• Power reset: Unplug the unit for 5 minutes, then plug back in and try again. Sometimes E1 is a temporary sensor read error.
• Locate the room sensor: Remove the front grille/filter. On most Keystone window and portable units, the room thermistor is a small probe near the evaporator coil or behind the air intake grille, connected by thin wires to the control board.
• Inspect wiring: Look for broken, pinched, or corroded wires and loose connectors between the thermistor and the main PCB. Reseat any loose plug connectors firmly.
• Check sensor resistance (advanced): With the unit unplugged, disconnect the thermistor from the board and measure resistance with a multimeter. At room temperature (~77°F / 25°C), most Keystone NTC thermistors read around 10kΩ (check service manual for exact spec). If it reads open (OL) or near 0Ω, the sensor is defective.
• Replace if faulty: If resistance is out of range or wiring is damaged, replace the room temperature sensor/thermistor assembly with a Keystone-compatible part. Route the new sensor in the same position away from direct cold air blast.
• Board check: If a known-good sensor still gives E1, the main control board sensor circuit may be defective and the board will need replacement.