Gas mileage will suffer slightly (nothing major.. 2-3mpg tops) after a
while. It can wait... no engine damage or anything will result.

-Mike

--
Melt away the Cellulite with Cellulean!
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If it's part of the emissions systems, it's covered by warranty to 50,000
miles.

Ummm, I don't think it is serious in my experience but your gas milage will
suffer, a little at first a lot later on and it may damage the catalytic
converter as well from what I understand, Please chip in with your info if I
am wrong I am new to working on my cars and am not completely certain as to
how it all works yet.

This happened on my 99 malibu after I didn't tighten the gas cap enough.
The service dept just reset the code, but told me that if I had opened and
retightened the cap and drove about 40-50 miles it would have reset itself.