Outdoor Unit Thermistor Open or Short
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 one of its temperature sensor circuits as open or shorted, such as the liquid pipe thermistor TH3, the outdoor ambient thermistor TH7 or the heat sink thermistor; which sensor is meant depends on the model. The unit stops the compressor because it relies on these readings for control and protection. 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 U4 shows on the wired remote.
- The code returns right after restart when a sensor is unplugged.
- The code appears after the outdoor unit was washed or after rain.
- No cooling while the code is active.
Possible causes5
- A loose or unplugged sensor connector on the outdoor board.
- Water or corrosion in the connectors after washing or rain.
- A sensor damaged by heat, sun and age.
- A cut or pinched sensor lead, or rodent damage.
- A fault in the sensor input circuits on the outdoor board.
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 identifies which sensor is meant from the board diagnostic LEDs or the error history, per the model's service manual.
- The technician checks and dries the sensor connectors and traces the leads for breaks or burns.
- The technician measures the suspect sensor's resistance against the manufacturer table, then checks the board if the sensors are good.
How to fix5
- Reseat, dry and clean the disconnected connector.
- Repair the damaged lead and secure it away from edges and hot pipes.
- Replace the failed thermistor with the correct part for the model.
- Replace the outdoor control board if its input circuit is proven faulty.
- Run the unit and confirm sensible sensor readings and no code.
Repair cost
In Saudi riyals; the highlighted part is the expected range for this fault.
The cost is low for a connector or a single thermistor, and rises 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
Which sensor does U4 refer to?
It depends on the model: it may be the liquid pipe, outdoor ambient or heat sink thermistor, and the technician identifies it from the diagnostic LEDs or the service manual.
How is U3 different from U4?
U3 is specifically the TH4 discharge thermistor, while U4 covers the other temperature sensors in the outdoor unit.
Can washing the outdoor unit cause it?
It can if water gets into the connectors, so the board and connectors should be covered during washing and allowed to dry before running.
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What was wrong? (optional)
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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