This topic takes me back. My Dad always said that whenever you’re fault-finding you start with the simple things and work towards the most complicated. About 40 years ago I was supposed to meet him but when I was part way there the oil pressure light came on. I found a phone box and called him in a panic.
“Is the oil level OK? Is it performing normally?”
“Yes.”
“Is the engine clattering or running hot?”
“No.”
“Then find a spares shop, buy a pressure switch for it and we’ll fit it when you get here.”
And that was it, of course.
Keith