Unit will not run, display shows E1 shortly after power-up or when trying to change modes.

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

Steps to check the room temperature sensor:

• 1. Power reset: Unplug the unit for 5 minutes, then plug back in and try again. If E1 returns, continue.
• 2. Access the front panel: Remove the front grille and filter. On many Koldfront window and through-the-wall units, the room thermistor is clipped near the evaporator coil or behind the air intake grille.
• 3. Inspect the sensor: Look for a small probe on thin wires clipped to the coil or in the air stream. Check for broken, pinched, or corroded wires and loose connectors going to the main control board.
• 4. Reseat connectors: Gently unplug and replug the thermistor connector at the control board to clear any oxidation or poor contact.
• 5. Test resistance (if you have a multimeter): With the unit unplugged, disconnect the thermistor and measure resistance across its leads. At room temperature (~77°F / 25°C), most Koldfront NTC thermistors will read in the 5–20 kΩ range. An open circuit (OL) or near 0 Ω indicates a failed sensor.
• 6. Replace if faulty: If the sensor is open/shorted or physically damaged, replace it with a Koldfront-compatible room temperature sensor/thermistor. Route the new sensor in the same location and clip it securely.
• 7. Control board check: If a known-good sensor still gives E1, the main control board’s sensor input may be defective and the board will need replacement.

When to stop: If you are not comfortable using a multimeter or removing the front cover, contact a qualified HVAC technician.