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Error Code
E0 means the indoor and outdoor units have stopped communicating. Each half runs its own board and passes settings and status between them, so when that link fails the system cannot coordinate and shuts down. On ActronAir equipment the comms pair carries low voltage, which makes it vulnerable to a poor connection long before anything expensive fails.
The interconnecting cable is the usual culprit on E0. A terminal that was never fully tightened, a core that has corroded in a coastal ceiling, or a cable pinched behind a casing will break the link. Because the comms signal is low voltage, even a slightly high resistance connection is enough to lose it.
Either board can fail on its communication port while the rest of it still works. The indoor board is the more sheltered of the two; the outdoor board lives with heat, rain and salt air and fails more often. A board is only condemned after the cable and the supply have been proved good.
Both boards need a stable supply to hold a signal. A voltage that sags when the compressor starts, or a supply shared with a large load, will drop the comms link and produce E0 intermittently. Repeat codes at the same time of day are the giveaway, and measuring under load confirms it.
Isolate both units at their isolators for a full two minutes, then restore power and watch the restart. A comms link that re-establishes and stays up suggests a transient event. One that drops again within minutes is a live fault, and repeated resets will not fix it or tell you anything new.
The interconnecting cable is checked terminal to terminal: tightness, corrosion, correct cores and continuity on each conductor. Any joint on the run is opened and inspected. This is the step that finds most E0 codes and it costs nothing but time, so it comes before any board is considered.
With the cable proved good and the supply stable, each board comms port is tested for signal and for the voltage it should be sending. A board that is silent on a circuit known to be sound has failed. Replacing a board before that test is how a good one gets thrown away.
