What it means
The indoor manuals list U codes only as "Outdoor unit failure" and refer engineers to the outdoor unit service manual. Mitsubishi's PUZ-WM service manual lists U4 as open or short of outdoor unit thermistors TH3, TH34, TH6, TH7 and TH8. Other outdoor unit ranges may differ.
What to check, in order
- Write the code down and reset once
Switch the system off, clear the code at the main remote controller and see if it returns.
- Call your installer
The checks are inside the outdoor unit: connectors, sensor resistance and the outdoor control board. Don't open the outdoor unit.
When to call your installer
Mitsubishi's PUZ-WM service manual lists disconnected or loose connectors, a faulty thermistor or a faulty outdoor controller board. It also notes that emergency operation is possible when some of these sensors fail.
A repair visit will usually check the sensor's reading and its wiring.
- Note the model and serial number. They're on a label on the rating plate on the outdoor unit, and on the indoor unit or cylinder.
- Write down the exact code, and what the appliance was doing when it appeared.
- Check whether it's still under warranty. If it is, the repair may cost you nothing.
Quick answers
What does the Mitsubishi Ecodan U4 error code mean?
One of the temperature sensors (thermistors) in the outdoor unit is reading as open or short circuit. The indoor manuals list U codes only as "Outdoor unit failure" and refer engineers to the outdoor unit service manual. Mitsubishi's PUZ-WM service manual lists U4 as open or short of outdoor unit thermistors TH3, TH34, TH6, TH7 and TH8. Other outdoor unit ranges may differ.
Can I fix Mitsubishi Ecodan U4 myself?
Engineer only (engineer visit). Start with: write the code down and reset once; call your installer. Mitsubishi's PUZ-WM service manual lists disconnected or loose connectors, a faulty thermistor or a faulty outdoor controller board. It also notes that emergency operation is possible when some of these sensors fail.
Is Mitsubishi Ecodan U4 worth repairing?
Worth fixing. A loose connector or a sensor is a modest repair. If the outdoor control board has failed, get a quote first.
Checked against
- Ecodan Hydrobox EHSC/EHSD series (D models) Installation Manual, error code table (manualslib copy)
- Ecodan Hydrobox EHSD/ERSD series (E models) Installation Manual DG79T728H01, error code table (manualslib copy)
- Ecodan PUZ-WM(50-60-85-112) Outdoor Unit Service Manual (OCH727A) (PDF)
Last reviewed October 2026. Manufacturers update their pages, and your manual is the final word. Spotted a mistake? Email contact@appliance-error-codes.com.