Keystone air conditioner shows E2 and may run briefly then shut off or not start cooling.

Safety: Disconnect power before opening the cabinet. Sharp metal edges are present around the coil and chassis.

Steps:

• Access the evaporator: Remove the front grille and filter. On window units, remove the front cover to expose the evaporator coil (the cold coil facing the room).
• Find the sensor: Look for a small probe or bullet-shaped sensor clipped or taped to the evaporator tubing with two small wires leading back to the control board.
• Check mounting: Ensure the sensor is firmly attached to the coil surface. If it has fallen off, re-clip or re-tape it securely to the coil in its original location.
• Inspect wiring: Look for cuts, pinches, or corrosion on the sensor wires and connector. Reseat the connector on the main PCB.
• Test resistance: With the unit unplugged, disconnect the sensor and measure resistance. Compare to the room sensor spec at similar temperature (often ~10kΩ at 77°F). If it reads open or shorted, replace the evaporator thermistor.
• Replace parts: Install a Keystone-compatible evaporator temperature sensor. If a new sensor does not clear E2, the main control board may be defective and require replacement.