Check Engine Light XJ1999 2 dr 4cyl

I've been having this problem for a while now and I keep feeling like the mechanics are yanking me around. My check engine light came on and ny gas milage went from 22 MPG to 14 MPG, so I went to my mechanic and pulled the codes. It said that both O2 sensors were out.

I got them replaced and the check engine light came back on after about 20 miles. I drove with the check engine light for about a week and noticed that the MPG were back to 20-22. I went back to the shop and pulled the code...same thing BOTH O2 sensors were failing. He suggested I needed to change my computer.

I went online, gave the jeep computer company my vin#, they flashed it, and I replaced the CPU. The check engine light comes on after about 15miles of driving. I go back to the mechanic and it still says that both O2 sensors need replacement.

My mechanic now suggests that it might be an electrical problem between my CPU and the engine. I check with the Jeep dealership and they don't even carry the cables (discontinued) from the CPU to the engine anymore.

The Jeep dealer wants me to bring it in, so he can charge $130 to check it out. In the same sentence he tells me that he might not be able to fix it with just the initial check. If it's an "eletrical problem" it may take 4 hours or so to find the problem.

I'm sorry for the long post, but I'm who should I believe. Does anyone have any insight on this? My Jeep drives absolutely perfect with the exception of the check engine light. I would totally appriciate any suggestions. Thanks Guys.

Don't worry about it. And yes they are yanking you around because they obviously don't have any idea what the real issue is. It sucks that you replaced your computer, b/c those are pricey. Prob. ran you about $300 right, assuming you got a new one, not refurbished?

I have the same issue on my 98. I get the codes for bad O2 sensors. I just replaced them and my jeep runs better and more efficiently, but I still get the code.

I'm not worried at all because there are no symptoms that anything is wrong. If the CEL really bothers you, go behind the dash and take out the bulb. I certainly wouldn't be taking it back to the shop though for them to throw more parts at it and waste your money.

Thanks for the reply...

Unfortunately, I do need to get the check engine light (CEL) light fixed because I have to get it smogged. I tried to get it smogged once when unplugging the CPU and then driving to the smog place (12/08). Even though the light was off, I failed smog and I noticed that it said something about "MLS Failure/Check Engine Light"

Any clue on what I should do now?

take it back to the original repair shop. you paid enough to have it "fixed" and now its on them. they didnt diagnose it right. you shouldnt have to pay for their mistakes

What numbers was the codes they pulled?

Post the exact codes here for comment. Be sure to check the fuse for the heater circuit on your oxygen sensors; should be a 15A fuse under the hood. As it is extremely unlikely that both new sensors are bad, I would perform a thorough inspection of the wiring for the sensors. Junk yards are a good (and cheaper) source of parts you can't get from a dealership.

These things can be kind of snaky to isolate. It doesn't sound like you're having the best luck with your old shop. I would ask them however if they would take another look at things (for no charge hopefully, at least for an hour or so as you paid them for 2 sensors you probably didn't need) to see if they can isolate. Or biting the bullet and taking it to a Jeep dealership may result in finding root cause quicker.

I'm not sure about the '99 but on my OBDII '96 if I disconnect the battery it resets the CEL that also has a mind of it's own. Since you only need to get it through smog and it seems to run fine you might give that a try. If it runs rough after you reconnect the battery the computer might need to 'relearn' by driving a few miles.
Good luck.

I'm not too sure how the smog tests work, but I would try just resetting the comp. or using a scanner to clear the code and then buying one of those bottles of w/e it is that guarantees you'll pass emissions tests if you use it.

If it doesn't work, you send in the UPC from the bottle saying it didn't make you pass and you get your money back.

As stated above, these kinds of things are very hard to track down. I explained that I'm having the same problem. I've replaced both O2 sensors and every single fuse and relay in the Jeep. I've replaced computers as well (I got a used one for a heck of a lot cheaper, though!; this was to solve another issue too), and still have the same problem.