I think my first step would be too go to the junkyard can get the parts you need so you can check the basic codes.

The wife's 940 had the same issue. I would do the battery trick and the light would stay off for a period, only to come back.

Turned out on her 94 it was the EGR vacuum solenoid. Very expensive part, so I tried a swap of the same part between my 240 and 940.

This was a spell back, and I think I swapped them back and forth couple of times.. and at some point it just ceased to be an issue. No more ck engine light.

So what happened? Not sure, but I would bet it related in some way to ground points or the ECM adapting in some way... maybe just the unplugging/reconnection of the part a number of times cleared up corrosion.

The one thing that is critical on all these older Volvos... especially when we start just buying new parts with no real change..is to get the corrosion and ground points all corrected.

I still need to do it properly on all three of my bricks.
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"Do you think that's air you're breathing now'? (The Matrix 1999) '94 940T (463K+), 92 245 (300K+), 90 740 (148k)