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Discussion Starter · #1 ·
I recently bought a used 2002 Elantra with ~80k miles on it. A few weeks ago, the check engine light came on, but the car seemed to be running fine. I got the code read at Auto Zone and it was P0139, they said likely the O2 sensor. Not abig deal, so I kept driving the car.

The light seemed to go on and off periodically. Then, a couple weeks later, while accelerating around ~ 50-55mph, the car would "clunk" and get stuck in 3rd gear and the light would immediately come on if it was off. Shutting the car off sometimes shut the light off and got it back to shifting properly.

I did as much research online as I could and found a lot of references to the Throttle Position Sensor (TPS). This sounded reasonable, so I replaced this sensor, but no luck.

Went back to Auto Zone this weekend and had it read again. This time it was the P0139 code again, but also the P1529 code (TCM MIL on request, check TCM for trouble codes).

It doesn't seem likely to me that the O2 sensor would be causing it to get stuck in 3rd gear, so I haven't replaced that - am I wrong? I've also read lately a lot about the speed sensors and similar gear problems. I don't really want to keep blindly replacing sensors, but don't really want to have to take it to a mechanic yet either. Any suggestions? Am I causing a lot more damage by driving it (I stay under 60mph / 3000-3500 rpm)?
 

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The o2 is not causing the trans problem. The code that will tell you why the trans is going into that limp home mod crap is being stored on the transmission controller. More than likely it is one of those speed sensors.
 

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Discussion Starter · #3 ·
Do I need to take this to a dealer to get the TCM code read? As far as I know the AutoZone tool won't read these codes. Can you buy the tool to read them (would it be cheaper?).
 

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any OBDII reader will read the code. The monkey running the scanner may not know what the code is for, but you can look that up for yourself. The dealer will have a scantool that will look at the status of the car when it is running that Autozone won't have.

And don't go to Autozone, they have a crappy corporate philosophy that makes life hard on their suppliers, and their prices are too high anyhow.
 
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