Mitsubishi Mini-Split Error Code U4: How to Fix It

Medium 30-60 minutes Medium Severity Verified June 2026
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Error Code
U4
Brand
Mitsubishi
Product Type
mini_split
Severity
Medium
DIY Difficulty
Medium
Estimated Fix Time
30-60 minutes
Error code U4 on your Mitsubishi mini-split indicates a short circuit in the outdoor coil thermistor. This typically means the thermistor is faulty or there's a wiring issue. Follow these steps to diagnose and fix the problem.
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Tools You'll Need

How to Fix Error Code U4

  1. Turn Off Power and Access the Outdoor Unit

    Always disconnect power before servicing. High voltage components can cause electric shock or death.

  2. Locate the Outdoor Coil Thermistor

  3. Test the Thermistor with a Multimeter

  4. Inspect Wiring and Connectors

  5. Replace the Thermistor if Faulty

  6. Reassemble and Test

Parts You May Need

NTC 10k ohm outdoor coil thermistor
Check Price on Amazon
NTC 10k ohm outdoor coil thermistor
Check Price on Amazon
NTC 10k ohm outdoor coil thermistor
Check Price on Amazon
NTC 10k ohm outdoor coil thermistor
Check Price on Amazon
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When to Call a Professional

If the thermistor tests good and wiring is intact, but error U4 still appears, the outdoor control board may be faulty. Contact a qualified HVAC technician for board replacement.

Frequently Asked Questions

What does error code U4 mean on a Mitsubishi mini-split?
Error U4 indicates that the outdoor coil thermistor has a short circuit. This can be caused by a faulty sensor or damaged wiring.
Can I still use my mini-split with error U4?
No, the system will typically shut down to prevent damage. The error must be resolved before normal operation.
How do I test the outdoor coil thermistor?
Use a multimeter set to ohms. At room temperature, a good thermistor reads around 10k ohms. A short reading (0 ohms) or open (infinite) means it's faulty.
Is the outdoor coil thermistor expensive to replace?
No, the thermistor itself is a low-cost part (typically under $30). However, if professional labor is needed, the total cost may be higher.
Where is the outdoor coil thermistor located?
It is attached to the outdoor unit's refrigerant coil, usually held in place by a clip or bracket. You may need to remove the top or side panel to access it.

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