U4

Open/Short Circuit of Discharge Temperature Thermistor (TH4)

Mitsubishi Electric · Mitsubishi PUZ-HA30-36NHA4 Outdoor Unit

What does U4 mean?

The discharge temperature thermistor (TH4) circuit is detected as either open (temperature reading 37°F [3°C] or less) or short (temperature reading 422°F [217°C] or more) during compressor operation. This fault prevents accurate monitoring of compressor discharge temperature, impacting system protection and control. Detection is inoperative for the first 10 minutes of compressor start-up and during/after defrosting.

Additional Steps

Steps 1–2 are listed in Priority Fix.

  1. 3

    Measure the resistance value of thermistor TH4 and compare it to expected values for the measured temperature.

  2. 4

    If TH4 and its wiring are confirmed correct, replace the outdoor controller circuit board.

Source: Mitsubishi PUZ-HA30-36NHA4 Outdoor Unit
Open Manual (Page 27)