I have the same problem, check engine light with no codes, no light at start, goes on in seconds, can't extinguish, car runs fine, no symptoms.

1998 S70 T5, 160,000 miles, not abused.

Recently did a number of maintenance actions, new alternator, ignition work, and some other tasks.

Upon startup had many error lights including: check engine, flashing up arrow, flashing transmission mode lights (sport, economy, winter). Found that the small red cable coming off the + battery post was loose. Tightened it and all lights went off on restart.

After 5 seconds the check engine light comes back on, no codes and my scanner is unable to extinguish the check engine light which it has done many times in the past.

Disconnected the battery cables overnight and the next day reconnected them, started the car, no CEL and then 5 seconds later it goes on. Again, no codes and my AutoXray scanner is unable to extinguish the light. Car runs fine. Transmission fluid level OK. Looked around to see if any other cables or sensor connectors had become dislodged and could find none. I even pulled the transmission and engine computers (with the battery cable disconnected) to make sure they would reboot. Not a solution.

(Other Background: I had a MAP sensor error some time back and replaced that, no current error codes. Had a vacuum leak error code sometime back, tightened the gas cap and no current codes. Recently installed a rebuilt ABS control module and the "Tracs" light is now off.)

Mark in MN
Three 98 S70 T5s, Dad, Daughter and Son