Mitsubishi Electric mini split error code U4
Outdoor unit temperature sensor open or short (TH3, TH6, TH7 or TH8).
- TypeFault
- SystemTemperature sensor
- Can I fix it?Homeowner checks, then maybe a tech
What U4 means on a Mitsubishi Electric
One of the outdoor unit's thermistors (outdoor pipe, outdoor air, heat sink or subcool) is reading open or shorted.
On this platform, U4 is a fault: something has failed or is disconnected and needs diagnosis.
Common causes
- A failed thermistor.
- A loose sensor plug on the outdoor board.
- A damaged sensor lead.
How to fix it yourself
Work through the checks below first; they resolve most cases. If they do not, the repair itself may need a technician.
- Reset the system: switch the breaker (or the outdoor disconnect) off, wait a full five minutes so the boards fully discharge, then switch it back on and restart from the remote.
- With power off, reseat every thermistor plug on the outdoor board.
- The outdoor board's LED blink pattern identifies which sensor; the service manual decodes it.
Switch off power at the breaker before opening either unit. Capacitors in the outdoor unit hold a charge for several minutes after power is cut.
When to call a technician
Sensor replacement is straightforward once the right one is identified.
Which Mitsubishi Electric units show U4
This page applies to Mitsubishi Electric M-Series (MSZ), Mr. Slim (PKA/PCA), Hyper-Heating H2i and MXZ multi-zone units. Mitsubishi Electric's own documentation lists U4 as "Outdoor unit temperature sensor open or short (TH3, TH6, TH7 or TH8)", the same definition used across the Mitsubishi Electric it is built on. If your unit shows a code with a different shape (for example a four-character code, or a plain number), it is from a different table; use the lookup with your exact code.
Where Mitsubishi Electric shows this code
Wall units without a wired controller signal a fault by blinking the operation indicator lamp; the code itself shows on a wired controller (MHK/PAR series) or on the wireless remote after entering its check mode (see your manual). On the outdoor unit, the control board LEDs blink the same code. P codes are indoor-unit faults, E codes are communication faults, F codes are outdoor-unit faults and U codes are outdoor-unit and compressor faults.
Other Mitsubishi Electric codes
Same part of the system as U4:
U4 on other brands
The same characters do not always mean the same thing. Check the brand on your unit.
Questions people ask
Can I reset a Mitsubishi Electric U4 code myself?
Yes. Turn the system off at the breaker for five minutes, then restart it. If U4 comes back, the fault is real and needs the checks above.
Is it safe to keep running a Mitsubishi Electric showing U4?
The unit will usually refuse to run, or run with reduced output, until the fault is cleared. Repeated resets do not fix anything and can mask a problem that is getting worse.
Do I need a technician for Mitsubishi Electric U4?
Work through the checks below first; they resolve most cases. If they do not, the repair itself may need a technician. Sensor replacement is straightforward once the right one is identified.
Sources
- Mitsubishi Electric M-Series and Mr. Slim service manuals (self-diagnosis tables)
Updated September 2026. Codes and definitions vary by model year. If your manual disagrees with this page, your manual wins. Send a correction.