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Error Code
E4 is the second of the ActronAir communication codes, and the practical difference from E0 is usually timing. Where E0 tends to be a hard failure, E4 often comes and goes: the system runs for a while, drops the link, resets itself and tries again. That pattern points at a connection that is marginal rather than a board that is dead.
An intermittent E4 is nearly always a joint that is not quite made. A terminal screw tightened onto insulation, a core corroded inside a connector, or a cable resting against a hot pipe will hold a signal most of the time and lose it under vibration or heat. These faults hide from a quick look.
A board with a failing comms port can work when cold and fail when it warms up, which produces E4 on a schedule rather than at random. The outdoor board is the more likely of the two given what it lives with. Substituting a known good board is sometimes faster than proving a fault inside one.
An unstable supply makes both boards reboot, and a reboot looks exactly like a lost comms link. Voltage that sags when the compressor starts, or a shared circuit with a large motor, will do it. A logger left on the supply for a day catches the dips a multimeter misses.
Isolate both units for two minutes and restore power. Then watch it run rather than walking away: how long it holds before E4 returns is the most useful information you will get. A fault that reappears at the same interval each time points at heat, or at a load switching on elsewhere in the property.
Every terminal on the interconnecting cable is opened, inspected and re-tightened, including the ones that look fine. Continuity is tested while the cable is flexed, because a broken strand inside intact insulation passes a static test. This is where intermittent E4 codes are usually found.
Each board comms output is measured and compared with what it should send. Where a board works cold and fails warm, the fault is confirmed by testing it in both states rather than once. That single distinction stops a good board being replaced on the strength of one reading.
