Mitsubishi Electric Error Code LED P1

Full Diagnosis

Diagnosis of malfunction: this is the LED blink-code equivalent of digital code P1, both flagging an abnormal reading (open or shorted circuit) from the indoor unit's room/intake air thermistor (RT1). It is used on indoor units or in service situations where no digital remote display is available, so the fault is read from a blinking pattern on the indoor unit's operation LED instead of a two-character code. Because the thermostat can no longer sense return air temperature accurately, the indoor unit halts normal thermostat-controlled operation until the sensor circuit is repaired; the fault is specific to that indoor unit and does not affect other indoor units on the same outdoor system.

All Recorded Causes

Room/intake thermistor (RT1) disconnected from the indoor control board. Corroded, oxidized, or loose thermistor connector pins. Thermistor wire chafed, pinched, or cut inside the indoor unit casing. Thermistor out of calibration or with a drifted resistance value. Moisture intrusion at the sensor connector. Failed indoor control board thermistor input circuit.

Verify Before Replacing Parts

1. Confirm the exact Mitsubishi Electric model family and default context before acting on a generic code description.
2. Verify the sensor or thermistor reading against the expected range for the current operating condition, keeping in mind that room/intake thermistor (RT1) disconnected from the indoor control board.
3. Check the sensor harness and board connection before condemning the sensor itself; on Mitsubishi Electric units this is also where you'd confirm corroded, oxidized, or loose thermistor connector pins.
4. After the correction, run the system long enough to confirm the default fault does not return under normal load.