I have a 2.9L (64K miles) that runs fine around town but misses at highway speeds. It starts missing about an hour into the trip and has done that consistently on three different occasions. From what I've read, it's most likely the O2 sensor.

What are the opinions here? Would my light come on if it was bad? I inquired at Auto Zone and the guy said if my check engine light wasn't on, he couldn't pull codes. Is that correct? What else could it be?

Thanks in advance!

 

1st: the Autozone thing. I haven't ever gone to Autozone to pull codes myself, but most of them don't seem to know anything about pulling codes on an EEc-IV system. 1st inaccuracy, you can pull codes on a 2.9 even when the check engine light isn't on. If there has been a problem, there will be a code in continuous memory (CM), and, if not, you will get pass codes. Then there's the fact that pre-88 2.9's didn't even have a check engine light to turn on (what would Autozone do there, never pull codes for you? Probably).
Fortunately, we aren't dependent on Autozone to pull codes. Read tests at www.dalidesign.com/hbook/eectest.html for instructions on pulling codes. You'll need a jumper wire (paper clip) and a voltmeter/test light. In the event you have a working check engine light, you can use the check engine light to output codes instead of using the voltmeter/test light.
I would pull codes first, before coming to any conclusions. If you post the codes here, please post with each code whether its from the KOEO, KOER, or CM portion of the test. The diagnostic tree for each code often depends on which type of code it is.

 

Thank you. Good info. Can I damage anything pulling codes myself? I'm not much of a mechanic. I mean... I can change a part if I know it to be bad. It's the troubleshooting that I have a problem with. I just wouldn't want to short anything out in the process.

 

I wouldn't be surprised if it's possible to damage something pulling codes yourself. If you follow the instructions, you'll be fine.