U8 • Mitsubishi Electric
Fault Code U8 Mitsubishi Electric: Causes, Diagnosis and Solution
Fault code U8 indicates an anomaly in the outdoor fan rotation speed during DC motor operation. This error is triggered if the rotation frequency is not detected according to specific thresholds related to the outdoor temperature.
Proof status : Manufacturer reference needed for the exact range
Source: Mitsubishi Electric documentation (service manual – according to product reference)
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Founder of AskMarcel.app. Sheets are written from manufacturer documentation; the reference is shown when it has been verified.
In brief
Fault code U8 on Mitsubishi Electric means: Fault code U8 indicates an anomaly in the outdoor fan rotation speed during DC motor operation. This error is triggered if the rotation frequency is not detected according to specific thresholds related to the outdoor temperature. Most common cause: The main cause is a malfunction of the outdoor fan DC motor operation or a failure of the outdoor controller board. First action: Check or replace the DC fan motor.. Call a registered engineer if the fault returns after basic checks or involves refrigerant work (F-Gas registered engineer required).
According to the Mitsubishi Electric service manual, fault code U8 is triggered if the outdoor fan speed is 100 rpm or less for 15 seconds at an outdoor temperature of 20°C or above.
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What does fault code U8 on Mitsubishi Electric mean?
Fault code U8 indicates an anomaly in the outdoor fan rotation speed during DC motor operation. This error is triggered if the rotation frequency is not detected according to specific thresholds related to the outdoor temperature.
The main cause is a malfunction of the outdoor fan DC motor operation or a failure of the outdoor controller board.
Symptoms
- Detection of a rotation speed of 100 rpm or less for 15 seconds at 20°C or above.
- Detection of a rotation speed of 50 rpm or less for 1 minute.
- Detection of a rotation speed of 1500 rpm or more for 1 minute.
- Display of fault code U8 on the outdoor unit screen.
Diagnosis
1. Mechanical and Electrical Check of the Motor
- Check or replace the DC fan motor.
- Check the voltage of the outdoor controller board during operation.
2. Check Parameters and Circuit
- Check if the stop valves are open.
- Check for loose, disconnected, or reversed compressor wiring.
Repair and return to service — Mitsubishi Electric fault U8
Start by checking or replacing the DC fan motor. If the problem persists after this intervention, check the voltage of the outdoor controller board during operation. If fault code U8 is still displayed, replace the outdoor controller board.
When should you call an F-Gas engineer?
Contact an F-Gas registered engineer if the fault returns after basic checks, involves refrigerant charging, or requires PCB or compressor replacement on Mitsubishi Electric.
Affected models
- Ecodan PUHZ SW50VKA BS
- Ecodan ATW PUHZ W50VHA BS
- Ecodan PUHZ HW140VHA2 BS
- Ecodan PUHZ HW140YHA2 BS
Field context
This code occurs during DC motor mode operation of the outdoor fan. It indicates that the unit cannot maintain a valid fan rotation speed according to the safety thresholds defined by the ambient temperature, thus preventing effective cooling or heating.
Frequently asked questions
What are the exact speed thresholds that trigger fault code U8?
Fault code U8 is triggered if the outdoor fan speed is abnormal according to two main criteria: a speed of 100 rpm or less detected continuously for 15 seconds when the outdoor temperature is 20°C or above; or a speed of 50 rpm or less, or 1500 rpm or more, detected continuously for 1 minute, regardless of temperature (Extract 1, Extract 2).
What is the first recommended repair step for a U8 code?
The first step is to check or replace the outdoor fan DC motor. If fault code U8 persists after this intervention, then check the voltage of the outdoor controller board during operation. If the problem remains, the controller board must be replaced (Extract 1, Extract 2, Extract 4).
Can a U8 code be diagnosed by checking the refrigerant?
Yes, although fault code U8 primarily concerns the fan motor, the manuals indicate checking for refrigerant leaks and short air cycles in the ducts as potential causes associated with the fan speed anomaly. It is also recommended to check the opening of the stop valves and the integrity of the compressor connections (Extract 3).
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