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Error Code
The U4 code means the indoor and outdoor units have stopped talking to each other. The system shuts down because it can no longer coordinate the compressor, fan and sensors, and running it blind risks damage. In practice the cause is almost always on the comms path between the two units, not in the refrigerant circuit. A technician works from the cheapest thing to test towards the most expensive, so the cable and the connections get checked before anyone quotes for a circuit board.
The two-core comms cable carries the signal that keeps the head and the condenser in step. Rodents in the ceiling, UV damage on an exposed run, a pinch at the wall penetration, or a screw through the cable during a renovation will all break it. It is the cheapest fault on this list to rule out, which is why continuity is tested end to end before a board gets quoted. A damaged cable usually fails outright rather than intermittently.
U4 shows when the outdoor board stops answering, not when it answers incorrectly. Loose terminal screws, corrosion on the connector pins from salt air, or a board that has dried out after years of heat cycling will each cause a dropout. Because a board is the expensive part, it gets tested last. Supply voltage and the comms cable are confirmed first, so a board is only replaced when it is genuinely the fault.
A comms run laid alongside mains cable, or a new inverter or variable speed drive on the same circuit, can corrupt the signal without breaking the wire. This usually presents as an intermittent U4 that clears on its own and returns under load. Separating the comms cable from power, or fitting shielded cable, costs a fraction of a compressor or a board. It is worth checking before parts are ordered.
A technician traces the comms cable from the indoor terminal block to the outdoor board, checking continuity and looking for rodent damage, crushed sections and corrosion at both ends. Exposed runs are checked for UV cracking. If the cable is damaged, it is replaced and the fault normally clears straight away. This is the first test because it is quick, non-invasive and the most common cause of U4.
With the cable ruled out, the outdoor board is inspected for burn marks, swollen capacitors, corrosion on the pins and loose terminal screws. The comms LED on the board is a quick indicator: no activity at all points at the board rather than the wiring. A board is only replaced after supply voltage and the cable have been confirmed good, because swapping a board over a wiring fault wastes both time and money.
Terminals at both ends are checked for tightness, correct polarity and corrosion, and the cable is confirmed to be the type and gauge the system expects. Polarity swapped at installation is a common cause on a recently installed unit, and it will present as U4 from day one. Once the connections are sound, the system is powered up and the code watched to confirm it does not return under load.
