Cause
Open circuit in relay sense wiring
A break in the wire by which a module confirms a relay actually switched. The module drives the relay on one circuit and watches the result on another — the sense line — so losing the sense line means the module cannot verify what it commanded.
The controlled device may work perfectly throughout. The relay closes, the load runs, and the module reports a fault because it cannot see it happen. A component that is plainly working with a circuit code stored against it is this pattern, and it is worth recognising before the relay or the load is replaced.
It also breaks a genuine safety check, since the sense line is how the module detects a relay welded closed. Test at the module's sense pin with the relay commanded on and off, comparing against the relay's output terminal — a voltage present at the relay and absent at the module names the wire. These are low-current circuits, so corrosion matters more here than on the supply side.
Typically a moderate job.
Codes this can cause (2)
Codes where this is a likely explanation first, rather than merely a possible one.
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