Question about 1999 Jeep Cherokee

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P0420 2003 Jeep Grand Cherokee 4.0 6cyl

First had error showing missfire in cyl 2 6 tech foud wiring harness puncture to yellow wire going to injectors repaired changed plugs , coil pack OK. Running beautifull now for 2 weeks . Yesterday error code p0420 popped up car still running fine . 420 seems to indicate catalytic converter problem. Wouldn't i be loosing power? Exhaust pipe blackened but may be due to original probs with cyl misfires. Can you advise me please. Thanks Much Fred

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This code is a catalyst efficiency code. The catalytic converter is most likely the problem, but it could still be caused by a defective oxygen sensor. The easiest way I know to check for this code is with a scan tool that is capable of reading live data stream. You need to look at the upstream and downstream oxygen sensors, and see if they are mirroring each other. In other words, if the upstream (the one before the catalytic converter) is reading .700, and the downstream ( the one behind the catalytic converter) is reading .700, or anywhere near that, then you have a problem. The numbers are just examples. The upstream should constantly swith from high to low voltage. If the downstream is identical, or even remotely close to having the same reading as the upstream, then you most likely have a defective catalytic converter. This does correspond to what happened a couple of weeks back. A misfire will take out a catalytic converter if not repaired quickly. Also, if you had a converter that was marginal, and was close to failure, then that misfire may have been just what it needed to fail.

Posted on Nov 10, 2008

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