95 grand 4.0L 2wd.

My friend just bought this jeep from a small car dealership, the jeep looks great and runs smooth. However on his way home cruising freeway speed the check engine light came on. He called the car lot and they had him go to a mechanic they use. He Parked it for 5 min, the guy went out with a scanner and there was no check engine light on and no codes or pending codes per the technician at that shop.

He gets back in the jeep and hops back on the freeway, check engine light comes on again. It has since went away and he has only driven it short distances and the light is off.

Does anyone have an idea to start troubleshooting this issue?

I personally have done tons of work to the 2 XJs that I had/have so I know my way around them but they dont have obd1 so that's why ill ask before I start looking into things.