Random Error with No Pattern: Drain Pump Cable Interference

Communication errors appearing at irregular intervals on a system where every component tests sound. Wiring, terminals and boards have all been checked more than once. The faults continue regardless.

When every part tests healthy and the fault persists, the useful question changes from what is broken to what is happening at the moment it occurs. Signal wiring carries very small currents and is sensitive to electrical noise from anything running close by. Noting what else operates when the fault appears is free, and it points at a relationship rather than a component. The drain pump cycling at exactly those moments is the observation that matters here.

A communication link carries very small signals, and a motor starting nearby produces electrical noise. Where the signal cable runs alongside that motor's supply, some of the noise couples onto the link and corrupts what is being sent along it. Both devices are working entirely correctly. The fault exists only in the relationship between the two, which is precisely why testing either one in isolation finds nothing at all.

The signal cable is separated from the pump's supply and screened where the route makes separation impossible, which stops the noise reaching the link. Nothing at all is replaced. The reasoning is worth carrying forward: when everything tests sound and the fault persists, stop looking for a failure and start noting what coincides with it. Relationships between two devices are entirely invisible to component testing.