Weird CEL issue

I have a strange issue with my 98 XJ. The check engine light came on after having crankshaft bearings, rear main seal, timing chain and NSS all replaced. The light stays on for 1-3 days then goes off for 1-2 days then back on. I had my technician check for codes but there no codes at all. Jeep runs great though. Maybe a little rough until it warms to operating temp. It seems very random except for it continuing. My tech is stumped too. Has anyone on here ever run into this issue?

That's really hard one to diagnose! Anything from a intermittent wiring short to a temporary out-of-spec sensor reading. I bought a scanner that has "freeze frame" that's stores the status at the moment when the cell comes on.

Weird that there aren't any retrievable codes, maybe while the inspection plate was off some crud got moved and kicked up to the cps by the flex plate? Why did you not do rod bearings too?

Sorry rod bearings were also replaced. Almost complete overhaul except for piston rings and head gasket. Still dealing with this issue. I’ll ask my tech about possibility of flex plate kicking up crud into CPS. Thanks bud!

Finally figured out what the issue was. The new NSS was defective and was feeding voltage back to the ECM so the computer recognized there was a problem but did not have a code for that issue. Older OBDII was limited like that I guess. My tech said it’s getting very difficult to find quality replacement parts these days.

I had my Chevy in the shop for some work they are having problems finding parts for the junk they are working on it's the world we live in.