Abnormal High Discharge Temperature or Insufficient Refrigerant

What it means

This code appears on the wired remote of Mitsubishi Electric Mr. Slim ducted, ceiling and cassette units. It means the discharge thermistor TH4 on the compressor outlet line recorded a temperature above the safe limit, or the unit concluded from its sensors that refrigerant is short. The unit stops the compressor to protect it, retries, and locks out with the code if it repeats. The same code can mean something else on other brands, and wall-mounted Mitsubishi Electric models show errors as lamp blinks with a different meaning.

Models covered: Mitsubishi Electric ducted, ceiling and cassette units (Mr. Slim) with check codes on the wired remote; wall-mounted Mitsubishi Electric models may show errors as lamp blinks with a different meaning.

Related codes: U3P8P6U1

Symptoms4

• Cooling gets gradually weaker before U2 appears.
• The outdoor unit stops after running for a while, especially in the heat.
• Frost on the small service valve or indoor pipes in some cases.
• Oily marks at pipe joints when there is a leak.

Possible causes5

• Low refrigerant from a leak at joints or coils, the most common cause.
• A partly closed service valve or a restriction in the circuit.
• A faulty electronic expansion valve restricting refrigerant flow.
• A false reading from the TH4 discharge thermistor because it is displaced or failing.
• Long running in very high outdoor temperatures with a dirty condenser.

How to diagnose5

1. Turn the unit off at the breaker and do not keep restarting it, so the compressor does not overheat.
2. Note whether cooling faded over weeks, which points to a leak.
3. The technician measures operating pressures and pipe temperatures against the manufacturer specification to detect undercharge.
4. The technician searches for the leak with a detector or bubble solution at joints and coils.
5. The technician checks the service valves are open, the electronic expansion valve works, and TH4 is seated and reading correctly.

How to fix6

1. Open the service valves fully or clear any restriction.
2. Refit the TH4 thermistor or replace it if its reading is wrong.
3. Have a licensed technician repair the leak, evacuate the system and recharge by weight.
4. Replace the electronic expansion valve if proven faulty.
5. Wash the condenser to lower operating temperatures.
6. Run the unit, watch discharge temperature and confirm the code does not return.

Repair cost

In Saudi riyals; the highlighted part is the expected range for this fault.

The cost is low for a sensor or closed valve, and rises for a leak repair with recharge or a new electronic expansion valve.

Prices are rough estimates including the part and the technician’s fee, and vary with brand, model and part source.
Prices last updated: September 2026

Frequently asked questions