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Mitsubishi Electric Air Conditioner U3 / U4 Error Code

Outdoor temperature sensor fault. What it means, which models show it, and how to clear it, easiest fix first.

Mitsubishi ElectricAll Mitsubishi Electric air conditioner codes

U3 / U4
May need a pro

One of the outdoor unit's temperature sensors (thermistors) has an open or short circuit.

Compatible models

Mitsubishi Electric doesn't publish a model list for this code; it appears on Mitsubishi Electric air conditioners with a digital display. Check your owner's manual.

Also shown as

Mitsubishi Electric lists U3 and U4 together for outdoor thermistor faults; which sensor each one points to depends on the outdoor unit series, so check its manual.

Difficulty

Try the basic checks, but the usual fix is a technician's job.

Safety first: unplug the air conditioner (or switch off its breaker) before touching filters, hoses or panels. If you smell burning or gas, or see water near the outlet, stop and call a professional.

How to fix Mitsubishi Electric U3 / U4

  1. Reset the power at the breaker

    Switch off the breaker that feeds the air conditioner (some systems have separate breakers for the indoor and outdoor units, so switch off both). Wait about five minutes, switch the power back on and restart the unit to see if the code clears.

  2. Note the full code and model number

    If the code returns, write down exactly what the controller shows and the model numbers from the labels on both the indoor and outdoor units. Outdoor sensors differ between series, so this lets the technician bring the right part on the first visit.

When to call a technician

The sensors are inside the outdoor unit, so replacement is a technician job.

Parts linked to U3 / U4

These are the parts most often behind this code. Tap one to see what it does and the signs it is failing.

Order parts by your full model number so they fit.

Frequently asked questions

What does U3 / U4 mean on a Mitsubishi Electric air conditioner?

One of the outdoor unit's temperature sensors (thermistors) has an open or short circuit.

Which Mitsubishi Electric models show the U3 / U4 code?

Mitsubishi Electric doesn't publish a model list for this code; it appears on Mitsubishi Electric air conditioners with a digital display. Check your owner's manual.

Can I fix Mitsubishi Electric U3 / U4 myself?

Only the basic checks. You can safely try the steps above, but U3 / U4 usually points to a part or fault that needs a technician.

What if U3 / U4 keeps coming back?

A code that returns after a reset and the checks above usually means a part is wearing out. Note when U3 / U4 appears (which cycle, how often) and have your model number ready; it helps a technician bring the right part on the first visit.

Other Mitsubishi Electric air conditioner codes

Last reviewed 2026-10-19. Codes and steps can vary by model; check your owner's manual.

Sources we checked