I hope this is the right place.
My brother who is not computer literate asked me to pose this question.
He just purchased a 2006 GMC Suburban no warranty. On the way home he noticed the oil pressure gage was pegged at the high mark. The engine seems to run fine, he has been driving it but wonders why the oil gage is pegged. Bad sending unit? Bad gage?

 

Bad sending unit? Bad gage?....Maybe, change the sending unit 1st...cheapest and the easiest ....maybe a bad ground. I take it all the other gauges are working OK.