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Discussion Starter · #1 ·
I hope someone can give me some advice. I have a 97 TJ 2.5l w/ 122k miles. I was getting 14-15 mpg. My transmission exploded so it sat for 4-5 months while I saved money. Fixed the transmission. The water pump when out on the way home from the shop. I fixed that but now I am getting 8 mpg. I am sure it needs a tune-up but that should the drop should not be that drastic or that fast. None of these things, except sitting for a long time, should have any effect on mpg.
I know the 2.5l is inherently sluggish but mine bogs from about 2200 it 3200 rpms then smooths out after that.

I have error codes. I am guessing 21 and 51 are my issues. Is this something that I can fix myself? My skillset is pretty good once diagnosed I can usually complete my own repairs.

12 Direct battery input to PCM was disconnected within the last 50 Key-on cycles.

21 Upstream oxygen sensor response slower than minimum required switching frequency. Upstream oxygen sensor heating element circuit malfunction. Downstream oxygen sensor heating element circuit malfunction. Downstream oxygen sensor input voltage maintained above the normal operating range. Oxygen sensor voltage too low, tested after cold start. (Upstream or Downstream) Left oxygen sensor input voltage maintained above the normal operating temperature.

51 A lean air/fuel mixture has been indicated by an abnormally rich correction factor.
 

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Have you crawled under there to look to see if any wiring or the O2 sensors were damaged/disconnected during the transmission repair? A malfunctioning O2 sensor can cause a rich environment and sluggish operation.
 

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My #1 suspicion is nothing more than your upstream 02 sensor has gone bad. 02 sensors go bad and like spark plugs, they need to be replaced every so often. Especially with that 21 code you're getting.
 

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Discussion Starter · #4 ·
And that should be located just ahead of the catalytic converter changed from the bottom, right? SHould i do both?
 

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You'll find the upstream 02 sensor on the downpipe, just below the exhaust headers. No need to replace the downstream 02 sensor located on the cat, it has nothing to do with this. The downstream sensor only monitors the catalytic converter and seldom goes bad.
 

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Discussion Starter · #6 ·
Great, thanks. Might as well do plugs, wires, cap and rotor while am at it.
i was wondering what i was going to do w/ my free afternoon this weekend.
 

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Make sure to only buy an NTK brand 02 sensor, avoid Bosch 02 sensors which are known to cause problems.
 

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Disco the battery's negative connectors when you start the job, that way the computer's 21 code will be cleared/reset by the time you're done and ready to start the engine.
 

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Discussion Starter · #12 ·
So I changed out the oxygen sensor, cap, rotor, plugs and wires and I am back to 13 mpg. It was nice driving w/o watching the needle free fall. The first 1/4 of the tank still seemed to go fast but then slowed down. I think the computer was slowly adjusting from 6-7 mpg to 13. I think the next tank of gas will be better but if not i will be happy with the 13.
 
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