Rough Idle/check engine light on -
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post #1 of 6 Old 06-30-2009, 09:47 PM Thread Starter
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Rough Idle/check engine light on

Hey Y'all,

My 03 grand cherokee has been running rough at idle, seems to shake the truck some. I also have a check engine light on. My local garage says I either have a bad O2 sensor or my catalytic converter is possibly clogged. How do I know which it may be?

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post #2 of 6 Old 06-30-2009, 09:54 PM
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check the code

Retrieving codes:

Turn the ignition key until the digital odometer displays, repeat three times in succession and then back to "On" (On/Off, On/Off, On/Off, On). At the fourth "On" the odometer will be replaced with codes. If no codes are present then "Done" will appear.

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post #3 of 6 Old 06-30-2009, 10:02 PM Thread Starter
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I am getting the same diagnostic code of p0420, could be an O2 sensor or the converter.
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post #4 of 6 Old 07-01-2009, 07:14 AM
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That code is thrown when the downstream O2 sensor signal oscillates at nearly the same frequency as the upstream sensor signal. Normally the downstream sensor oscillates more slowly. It is very, very unusual for an O2 sensor to go bad in such a way to trigger the P0420 code. Because that just doesn't happen, the manufacturers use that method to gauge catalytic converter efficiency. You need to have the cat replaced.

With the right equipment, a mechanic can display the signal voltages from both the upstream and downstream at the same time and confirm that it is or not an O2 sensor problem. You might want to have that done before the cat is replaced, but there will be a cost for the diagnosis.

Bank 1 is the bank of cylinders on the driver's side in the V8. For the I6, it's the front three cylinders.
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post #5 of 6 Old 07-01-2009, 04:36 PM
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im having the same problem but my cel is not on. and im getting a very dark black soot building up on my tail pipe
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post #6 of 6 Old 07-01-2009, 04:52 PM
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try cleaning your idle air control vavle. you can buy cleaner at your local auto parts store, and the valve is on the right side of the TB when looking at the engine from the front.
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