In brief

Fault code U3 on Mitsubishi Electric Ecodan PUHZ SW50VKA BS Service Manual OCH525C — ecodan-puhz- means: Fault code U3 indicates an open circuit or short circuit detected on the discharge temperature thermistor (TH4) and/or the compressor surface thermistor (TH34) during compressor operation. Detection is inactive for the first 10 minutes of compressor start-up and during or after defrost. Most common cause: The main cause is a disconnection or contact failure on the connector (TH4/TH34) of the outdoor controller board, a faulty thermistor, or a faulty outdoor controller board. First action: Inspect the connector connection (TH4/TH34) on the outdoor controller board.. 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 for the Ecodan PUHZ SW50VKA BS, fault code U3 is triggered by an open circuit (≤3°C) or short circuit (≥217°C) on the discharge temperature thermistor (TH4) and/or compressor surface thermistor (TH34).

U3 • Mitsubishi Electric • Ecodan PUHZ SW50VKA BS Service Manual OCH525C — ecodan-puhz-

Fault code U3 Mitsubishi Electric: causes, diagnosis and solution

Fault code U3 indicates an open circuit or short circuit detected on the discharge temperature thermistor (TH4) and/or the compressor surface thermistor (TH34) during compressor operation. Detection is inactive for the first 10 minutes of compressor start-up and during or after defrost.

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

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What does fault code U3 on Mitsubishi Electric mean?

The main cause is a disconnection or contact failure on the connector (TH4/TH34) of the outdoor controller board, a faulty thermistor, or a faulty outdoor controller board.

Symptômes

Display of fault code U3 on the indoor unit.
Compressor stops due to overheat protection or measurement fault.
Specific blinking pattern on the outdoor unit board LED (e.g., 1 flash every 2.5 seconds on some models).

Diagnostic

1. Check connections

Inspect the connector connection (TH4/TH34) on the outdoor controller board.
Check the integrity of the conductor wire for the thermistor (TH4/TH34) for any breaks.

2. Measure components

Measure the resistance value of the thermistor (TH4/TH34) or check the temperature displayed by the microprocessor.
Use the outdoor unit monitoring function (SW2 on the A-Control service tool) to confirm the reading.

How to reset fault code U3 on Mitsubishi Electric?

If the connection and wire are intact, replace the faulty thermistor. If the resistance measurement is correct but the fault persists, replace the outdoor controller board.

When should you call an F-Gas engineer?

This code appears during diagnosis of a Mitsubishi Electric Ecodan air conditioning fault where the compressor does not start or stops mid-cycle. The engineer must wait for the defrost cycle and the 10 minutes post-start-up to conclude a hardware fault.

Affected models

Ecodan PUHZ SW50VKA BS
Ecodan ATW PUHZ W50VHA BS
MUYTP3550VFS
Ecodan PUHZ HW140VHA2 BS
Ecodan PUHZ HW140YHA2 BS

Field context

Technical source data

Fault code U3 signals an open circuit (≤3°C) or short circuit (≥217°C) on the discharge temperature thermistor (TH4) and/or compressor surface thermistor (TH34, TH33 depending on model) during compressor operation. Detection is inactive for 10 minutes after compressor start-up and during/after defrost. Causes include a disconnection of the connector (TH4/TH34) on the outdoor controller board, a faulty thermistor, or a faulty controller board. Verification should focus on wire continuity and thermistor resistance value (Extracts 1, 2, 4). Note that some manuals also mention TH3, TH6, TH7, TH8 for this code (Extract 5), although the main fault targets TH4/TH34 for Ecodan SW50VKA models (Extract 2).

Sources: Ecodan ATW PUHZ W50VHA BS Service — ecodan-atw-puhz-w50vha-bs-service.pdf (p. 16) · Ecodan PUHZ SW50VKA BS Service Manual OCH525C — ecodan-puhz-sw50vka-bs-service-manual-och525c.pdf (p. 22) · MUYTP3550VFServicemanualOBH817C — muytp3550vfservicemanualobh817c.pdf (p. 24)

Questions fréquentes

Why does code U3 not appear immediately after compressor start-up?

Detection of code U3 is intentionally inactive for the first 10 minutes of the compressor start-up process and during and after the defrost cycle. This safety measure prevents false positives due to transient thermal variations. The engineer must wait for these periods to end before carrying out resistance or continuity checks on the thermistors.

What are the exact temperature thresholds for triggering a U3 fault?

Fault U3 is triggered if an abnormal value is detected during compressor operation: an open circuit corresponding to a temperature of 3°C or less, or a short circuit corresponding to a temperature of 217°C or more. These thresholds apply to the discharge temperature thermistor (TH4) and the compressor surface thermistor (TH34/TH33).

What is the diagnostic procedure if the connector connection is correct?

If the connector connection (TH4/TH34) on the outdoor controller board and the integrity of the wires are confirmed, measure the resistance value of the relevant thermistors. If the resistance is correct but the fault persists, the cause is likely a faulty outdoor controller board, requiring its replacement. Consult the test point diagrams in the service manual.

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