My check engine light is constantly coming on but no problem can be found. on 2001 Volvo S60

The check engine light has come on and off for about the last 10,000 miles; vehicle has 105,000 miles. Major tune-up has been done and car has been checked several times and codes reset only to have light appear again. Amount of fuel in tank does not seem to have any correlation to light. After last service am detecting a very strong gasoline odor but no evidence of leak. Service included a gasoline additive and last time I returned vehicle because of odor all gasoline was changed to clean out tank. Odor persists. Check engine light on again. Big puzzle.

Asked by for the 2001 Volvo S60
It sounds to me you keep having the light reset, but I didn't read in your question anything that leads me to believe corrective action or diagnosis has taken place. If you would, could you post the question again with a code number included? That would put me in the proper path to help you solve your puzzle.
Dealer said upgrade was installed and their invoice shows "installed 30677021-5 :ECM gasoline upgrade". Hope this helps. Only number I could find and hope it is "code number".
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There are more questions we will have than answers, but first of all, where are you taking it to be checked out? Is it the Volvo Dealer? Is it a shop that specializes in European cars? Somewhere else?
Usually to shop specializing in Volvo since dealer in my area closed. However, yesterday it was at a dealer to replace leaky fuel pump (extended warranty) and I was told they could check out the check engine light. After changing/upgrading codes and spending $100 light is on again; only drove 69 miles before it reappeared. Dealer says they will look at it again but it is extremely inconvenient for me to take it to them so may have to wait till next year. Meanwhile I'll just ignore it as car drives fine.
I agree with ABC below...if you can post any diagnostic info from the repair orders we may be able to provide some additional help.
there was a recall on the fuel pump gaskets and also i am not sure if you vehicle is the same as our volvo but if you dont take your keys out of the ignition before u pump gas it will thro a code