O2 related service engine light

A buddy of mine related to me that his 98 QX4 (90k miles) service engine light would come on periodically. He took it to Autozone to have the diagnostic code read. They told him he a bad O2 sensor. He replaced the sensor and a week or two later the service engine light came on again. To make a long story short, it turned out he had a bad gas cap. A new cap from Auto zone corrected the problem. About 2 months ago I had the same problem with my 97 QX4 (82k miles). Same fix, a new gas cap. Moral of the story:rather than replace an O2 sensor, you may want to begin w/ the gas cap.

My 2001's check engine light was on for 2-3 months before I finally took it to the dealer (after Goodyear said I had a bad O2 sensor and charged me $20-40 though they didn't fix anything). The dealer fixed the problem, though now I can't remember what was wrong with it though I know it wasn't the gas cap. They didn't charge me because it was covered by a recall of some sort.

Moral of the story:rather than replace an O2 sensor, you may want to begin w/ the gas cap.

I think the moral of the story is not to listen to Autozone give advice, get the actual P-codes and post here for advice. For your vehicle there are 70+ possible P-codes all with various causes, yes a few can be caused by a leaking gas cap but most are not.

I think the moral of the story is not to listen to Autozone give advice, get the actual P-codes and post here for advice. For your vehicle there are 70+ possible P-codes all with various causes, yes a few can be caused by a leaking gas cap but most are not.
Does Autozone's diagnostic checker have the ability to give the P-code or do you have to go to the dealer?

Autozone uses an OBDII reader for the 96+ vehicles and they all provide the same type of P-codes, same as what the dealer will read. I've had them read a shift ratio error and recommended a transmission rebuild when it was a 5 minute throttle position adjustment.