Outdoor Discharge Thermistor (TH4) Open or Short

Home Appliance Error Codes Mitsubishi Electric Ducted (concealed) AC

Can you still use it? With care until repaired

Who can fix it? Needs a technician

Typical repair time 30 minutes – 3 hours

Typical cost SAR 80 to 2,500

What it means

This code appears on the wired remote of Mitsubishi Electric Mr. Slim ducted, ceiling and cassette units. It means the outdoor board sees the TH4 discharge thermistor circuit as open or shorted, so it cannot monitor the compressor outlet temperature. The unit stops the compressor because it cannot protect it from overheating without this reading. 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.

Symptoms4

The outdoor unit stops and U3 shows on the wired remote.
The code returns right after restart or within a few minutes.
The code appears after work on the outdoor unit or after washing it.
No cooling at all while the code is active.

Possible causes5

A loose or unplugged TH4 connector on the outdoor board.
A thermistor damaged by repeated high temperature on the discharge line.
A sensor lead burned by touching the hot discharge pipe, or cut.
Water or corrosion in the connector after the outdoor unit was washed.
A fault in the sensor input circuit on the outdoor board (less common).

How to diagnose5

Switch off at the breaker for several minutes, restart and note when the code returns.
Switch off at the breaker before opening any panel; the outdoor unit and its inverter board keep a dangerous charge for a while after switch-off.
The technician checks the TH4 connector on the board and traces the lead along the discharge line for burns or breaks.
The technician measures the thermistor resistance and compares it with the manufacturer table at the current temperature.
If the thermistor is good and the code stays, the input circuit on the outdoor board is checked.

How to fix5

Reseat, dry and clean the connector.
Repair the lead and secure it away from hot pipes.
Replace the TH4 thermistor with the correct part and fix it firmly to the discharge line.
Replace the outdoor control board if its circuit is proven faulty.
Run the unit and confirm a sensible discharge temperature reading and no code.

Repair cost

0–200

200–600

600–1,500

1,500+

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

The cost is low for a new thermistor or lead repair, and rises sharply if the outdoor unit needs a new control board.

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

Is U3 a compressor fault?

No. It concerns the TH4 thermistor, its lead or its circuit; the compressor only stops because its temperature cannot be monitored.

Why does this sensor fail more than others?

It sits on the discharge line, the hottest point in the circuit, so in summer heat the sensor and its lead are under more thermal stress.

Can the unit run temporarily without it?

No. Bypassing the sensor removes the compressor's overheat protection and can destroy it.

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This guide is for information only and does not replace an inspection by a qualified technician. If the fault involves brakes, steering, engine temperature, electrics or refrigerant, do not attempt the repair yourself.

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