Mitsubishi Electric U2

What does Mitsubishi Electric U2 mean?

The discharge temperature at the compressor outlet climbed too high, which the control reads as overheating from a shortage of refrigerant. The compressor is stopped and U2 appears on a blinking display. Low charge means the refrigerant returning to the compressor cannot carry away enough heat, so discharge temperature rises past the protection limit.

Symptoms

• U2 shown on a blinking display after the compressor stops
• Weak cooling or heating performance
• Compressor short-cycling on the protection device
• Hotter-than-normal discharge line at the outdoor unit

Common causes

• Refrigerant shortage from a leak or undercharge
• Restricted refrigerant flow in the circuit
• Discharge thermistor (TH4) reading high or faulty
• Expansion device (LEV) not opening correctly

Diagnostic steps

1. 01

   Stop and reset

   Power the unit down, wait a few minutes, then restart and see whether U2 returns under load.

2. 02

   Look for signs of low charge

   Check for oil traces at joints or icing patterns that suggest a refrigerant leak or shortage.

3. 03

   Confirm airflow first

   Rule out airflow problems on both coils, since poor heat transfer can also drive discharge temperature up.

4. 04

   Note conditions when it trips

   Record whether U2 occurs in cooling, heating, or after a defrost to help the technician narrow the cause.

5. 05

   Arrange a charge and leak check

   A technician should measure discharge temperature and pressures and leak-test the circuit.

When to call a professional

Because U2 points to a refrigerant shortage, call a certified technician to leak-test, repair, and recharge the system. Locating leaks and setting the correct charge require gauges, a leak detector, and refrigerant-handling certification. Do not add refrigerant yourself; an incorrect charge can repeat the fault or damage the compressor.