I have a Setrab oil cooler with fan. The last few runs, the oil temperature was getting hot. Replaced the relay, but no help. Checked the wires and all was good. I tested the inline thermo fan switch and it was bad, so I replaced it with a new one that I also tested and the new one was good. Will take it for a run on Saturday. Didn't think those thermo fan switch would fail.

What Brand ?

Setrab

Anything mechanical can and will eventually fail.
Probably a broken contact in the switch.

Zach