Keystone air conditioner displays E3 and compressor may not start or shuts down quickly.

Safety: Unplug the unit. For window and through-the-wall models, slide the chassis out of the sleeve only if you can safely support it.

Steps:

• Access condenser area: On window units, remove the outer case to expose the rear (hot) condenser coil and fan.
• Locate sensor: Find the small sensor clipped to the condenser coil or discharge line, with two wires going to the control board.
• Check attachment: Ensure the sensor is firmly clipped to the coil or line. Re-secure if loose using the original clip or aluminum tape rated for refrigeration use.
• Inspect wiring: Look for chafed insulation where wires pass through metal, and for loose or corroded connectors. Repair any damaged sections and reseat connectors.
• Measure resistance: Disconnect the sensor and test with a multimeter. Compare to expected NTC values (similar to other thermistors). Replace if open/shorted or far out of spec.
• Control board check: If a new sensor and good wiring still result in E3, the main PCB is likely faulty and should be replaced as an assembly.