Check Engine Light and no code with S70

Couple months ago I brought my 1998 S70 to a good independent garage
that's specialized in European cars (esp. Volvo) due to persistant
Check Engine Light.

After being charged $70 for his diagnostic, I was told no code was
found. The light was simply reset and a new gas cap installed (they
found it leaky).

However, a couple weeks afterward the light came back. It actually went
off by itself once but after another couple hundred miles, it came
back. Also the light seemed to more often came up (after I reset it
with a OBDII reader) when the gas tank was near empty.

I'm afraid of being charged another $70 for nothing- at the garage or
dealer.

Anyone knows what's going on? My cheap OBDII reader can't read any code
either.

Appreciate any helps!

Ehien

 

I had repeated checklight problems on my 2000 S-40. The code said it was a gas cap
problem, and the mechanic blamed it on the gas cap not being put on tight. The third time
it happened, I was much annoyed, and the service manager suggested trying a new gas cap.
He took one out of the parts room, and as he put it on he said it was a slightly different
design. It fixed the problem. I suspect Volvo had a reason for the design change.