What does the Daikin U4 error code mean?

U4 is a communication error between the indoor and outdoor units. It means the two boards are not exchanging signals properly, which can be caused by a faulty PCB, damaged wiring, or a loose connection.

Can lightning damage just one part of the aircon system?

Yes. Power surges often affect only the component closest to the mains entry point. In split systems, the outdoor unit connects directly to the isolator and typically takes the hit first. The indoor unit may be completely unaffected.

Should I install a surge protector for my aircon?

A surge protector on the aircon circuit breaker adds a layer of defence against voltage spikes from lightning or grid fluctuations. It is especially worth considering if your area experiences frequent thunderstorms.

Daikin U4 PCB Failure After Lightning Surge

Aircon case in Pasir Ris, Singapore: electrical/control traced to outdoor PCB communication circuit damaged by power surge after targeted diagnosis checks.

Case Details

Reported: The aircon stopped working after a big thunderstorm last week. There is an error on the display and power cycling does not clear it. Someone told us both the indoor and outdoor units need to be replaced.
Unit: Daikin · Wall-mounted · 7–12 years
Location: HDB · Pasir Ris, Singapore

What We Checked

U4 communication error was displayed on the indoor unit.
Power cycling did not clear the error.
Indoor PCB was receiving power and responding to commands normally.
Signal voltage measurement on the S-terminal communication line showed a flat reading from the outdoor board — no transmission.
Outdoor unit showed no other visible damage — capacitor, compressor wiring, and fuses appeared intact.

The Diagnosis

The thunderstorm caused a power surge that damaged the communication circuit on the outdoor PCB. The outdoor board stopped transmitting its signal on the S-terminal line, which is how the indoor and outdoor units coordinate. The indoor PCB was unaffected — it was still sending and receiving normally on its end. The Daikin U4 error triggers specifically when indoor-outdoor communication fails, which is consistent with the outdoor board going silent.

What Fixed It

We explained that the fault was isolated to the outdoor PCB communication circuit. The indoor PCB, compressor, and all other components tested normal. Replacing the outdoor PCB would restore communication and clear the U4 error. A full system replacement was not necessary. We also recommended installing a surge protector on the aircon circuit to reduce the risk of future lightning damage.

The outdoor PCB was replaced. Communication between indoor and outdoor units resumed immediately, and the U4 error cleared. The system has been running normally since. A surge protector was installed on the aircon circuit breaker.

Why This Happens

Why lightning damage does not always mean full system replacement.

Power surges from lightning typically travel through the mains supply and hit the outdoor unit first, since it connects directly to the isolator.
The communication line between indoor and outdoor boards carries a low-voltage signal — measuring that signal isolates which PCB failed.
Replacing only the damaged PCB costs a fraction of a full system replacement and restores normal operation when the rest of the system is intact.

Same situation with your aircon?

Describe what’s happening. We’ll work out the likely cause and tell you the right next step.

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