Jumping back in here, my advice about the gas cap did not fit the original code info, but it was a continuous source of CE lights. I simply disconnect the battery ground side for 10-15 min, then reconnect and reset the radio. Even after that it still comes on occasionally. Point is, there is no cost to the elimination of the light and I will always be able to get thru inspection.

If the car runs ok, and inspection is accomplished, why spend money on it?
And how can you sell it with a CE light on?

The advice on measuring the ground strap voltage drop is very good. However the number you get depends on the current at the time. For any analysis, you need to describe the exact list of what is on and what is not. Best to have many things on. Did the poster say the engine should be running? Don't try to measure low resistance with a DVM. They cannot give an accurate reading.

I saw the words saying lubricate the bolts on the ground straps. I think that is bad advice. Remove and clean them for sure, along with the mating surfaces. Then apply a VERY THIN coat of Dielectric Grease. Maybe that is what the poster meant. The grease is conductive but the feature is that it prevents any oxygen from getting into the joint. That prevents oxidation.

Next is the vacuum theory. I have heard that there is a vacuum hose running to the charcoal cannister. Maybe look there? Also heard that spraying something (WD40, carb cleaner, ??) on joints will pick up leaks. How about the brake booster hose and valve? I realize these things do not match the codes described but are just tips on searching.

Finally, if the car runs OK and inspection is solved, just run it.

Sixvolvos, one of which is a '98 S90 at 150K miles.