In brief

Fault code U6 on Mitsubishi Electric Ecodan PUHZ SW50VKA BS Service Manual OCH525C — ecodan-puhz- means: Fault code U6 indicates a power module anomaly detected during an overcurrent test (UF or UP), or a short circuit at the module output interface (IC700). It may also relate to a connection fault of the discharge and compressor surface thermistors. Most common cause: The main cause is a short circuit at the power module output interface (IC700) or a short circuit in the compressor windings. Other causes include reduced supply voltage, faulty U-V-W phase connection to the compressor, or a defective component (power module, power board, compressor). First action: Check if the outdoor stop valve is closed and open it if necessary.. Call a registered engineer if the fault returns after basic checks or involves refrigerant work (F-Gas registered engineer required).

According to Mitsubishi Electric service documentation, the U6 code can be triggered by a closed outdoor stop valve or a supply voltage drop, as noted in extracts from the service manual.

U6 • Mitsubishi Electric • Ecodan PUHZ SW50VKA BS Service Manual OCH525C — ecodan-puhz-

Fault Code U6 Mitsubishi Electric: Causes, Diagnosis and Solution

Fault code U6 indicates a power module anomaly detected during an overcurrent test (UF or UP), or a short circuit at the module output interface (IC700). It may also relate to a connection fault of the discharge and compressor surface thermistors.

Source: Mitsubishi Electric documentation (service manual – according to product reference) · Updated on 16 June 2026

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What does fault code U6 on Mitsubishi Electric mean?

The main cause is a short circuit at the power module output interface (IC700) or a short circuit in the compressor windings. Other causes include reduced supply voltage, faulty U-V-W phase connection to the compressor, or a defective component (power module, power board, compressor).

Symptômes

Display of fault code U6 on the indoor/outdoor unit.
LED flashing on the outdoor power board: 6 times for 2.5 seconds, then 2.5 seconds off.
Unit stops operating.
Overcurrent detected when the power module is activated.
Abnormal resistance measurement on thermistors TH3, TH4, TH6, TH7 or TH34.

Diagnostic

1. Check Connections and Valves

Check if the outdoor stop valve is closed and open it if necessary.
Inspect the thermistor connectors (TH3, TH4, TH6, TH7, TH34) on the outdoor control board and connector CN3 on the power board to ensure they are not disconnected.

2. Check Supply and Wiring

Check the electrical installation and supply voltage.
Check the compressor connection for any looseness, disconnection, or phase reversal of U, V, and W.

3. Measure Components

Measure the resistance value of the outdoor thermistors.
Check the compressor and power module by following the component test procedures (refer to sections 8-4, 9-4 or 10-6 of the manual).

How to reset fault code U6 on Mitsubishi Electric?

If the stop valve was closed, open it. Correct any wiring or supply voltage anomalies. If the thermistors are faulty, replace them. If the compressor or power module is faulty, replace the outdoor power board or outdoor control board (depending on the SW50VKA or SW40/50VHA model) and check the compressor.

When should you call an F-Gas engineer?

This code appears during start-up or operation of the outdoor unit if the power module detects excessive current or a short circuit. The engineer must check the stop valve status, the integrity of the electrical supply, the phase polarity to the compressor, and the

Affected models

Ecodan PUHZ SW50VKA
Ecodan PUHZ SW40/50VHA
Ecodan PUHZ HW140VHA2
Ecodan PUHZ HW140YHA2
MUYTP3550VFS
Ecodan ATW PUHZ W50VHA

Field context

Technical source data

The U6 code signals a power module anomaly detected during an overcurrent test (UF or UP conditions) or a short circuit at the module output interface (IC700) and compressor windings. Causes include a closed outdoor stop valve, supply voltage drop, loose, disconnected, or reversed connections (U-V-W phases), or defective components (compressor, power board, control board). For SW50VKA models, a fault may also involve the discharge thermistor (TH4) and compressor surface thermistor (TH34) or their connectors (TH3, TH4, TH6, TH7, CN3). (Extract 1, Extract 2, Extract 3, Extract 5)

Sources: Ecodan ATW PUHZ W50VHA BS Service — ecodan-atw-puhz-w50vha-bs-service.pdf (p. 17) · Ecodan PUHZ SW50VKA BS Service Manual OCH525C — ecodan-puhz-sw50vka-bs-service-manual-och525c.pdf (p. 23) · MUYTP3550VFServicemanualOBH817C — muytp3550vfservicemanualobh817c.pdf (p. 24)

Questions fréquentes

What should I do first if code U6 appears?

Immediately check if the outdoor stop valve is closed; if so, open it. Then inspect the electrical supply for any voltage drop. Correct any wiring issues (looseness, disconnection, or phase reversal of U, V, W) to the compressor by referring to the test point diagram on the outdoor power board.

How do I differentiate between replacing the power board and the control board?

The procedure depends on the specific model. For units in the SW40/50VHA range, replace the outdoor power board. For SW50VKA units, replace the outdoor control board. Before doing so, ensure you have already ruled out wiring, voltage, compressor, and thermistor faults.

Can code U6 be related to thermistors?

Yes, on some models like the SW50VKA, code U6 can indicate an open or short circuit fault of the discharge thermistor (TH4) and compressor surface thermistor (TH34). Check that the connectors (TH3, TH4, TH6, TH7 on the control board and CN3 on the power board) are not disconnected and measure the resistance of the outdoor thermistors.

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