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Discussion Starter #1
Hello, just recently at long red lights my rpms drop and engine idles rough truck idles fine in park. I went through and change my plugs and ignition coils and have had no change. Yesterday I got the dreaded P0420 code, could the cat and the rough idle be tied together some way?
 

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Mambeau / Admin
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Hello, just recently at long red lights my rpms drop and engine idles rough truck idles fine in park. I went through and change my plugs and ignition coils and have had no change. Yesterday I got the dreaded P0420 code, could the cat and the rough idle be tied together some way?
Yes - your rough idle may well be connected to whatever is causing the P0420 code to be thrown. The code is mostly likely caused by a problem with:

- A gas leakage in the exhaust system
- The A/F sensor (Bank 1 sensor 1)
- The Heated O2 sensor (Bank 1 sensor 2)
- The catalytic converter

... any or all of which would screw with the ECM's ability to maintain a stable fuel trim.

Here (attached) is the 01-04 FSM section on the P0420 code.
 

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Mambeau / Admin
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Thanks! If it was one of the 02 sensors wouldn't i get another code on top of the P0420?
Maybe, but not necessarily ...

There are separate codes that get thrown if the ECM detects a condition indicating a specific sensor is FUBAR (e.g., null reading; circuit fault; reading wildly out of range).

However ...

It's possible for one or both sensors to misbehave and trigger a P0420 even though its / their individual performance is not screwy enough to trip one of the sensor-specific codes.
 

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Discussion Starter #5
So an update on the current situation...

Started the truck up yesterday and my son forgot something in the house so I had to turn it off to use my keys, i come back out go to turn it on and it turned over and died right away....The only way I could get the truck to turn on and continue to run was if I had my foot on the accelerator, as soon as i let off it would die. I did some online research and it brought me to the IAC on the throttle body, i watched a video that said to remove the main vacuum line on the TB and see if the engine would start up, lo and behold it did (obviously only for a few seconds due to vacuum line off) I am thinking this IAC is what has been causing the very rough idle and low rpms at red lights, I am leaning towards just replacing it and not cleaning...am I on the right track?


Thanks!!
 

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Mambeau / Admin
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Just for the record ...

Once you got the engine fired up again, did it respond to the accelerator OK? If so, this would probably rule out the Throttle Position Sensor (TPS) as part of the problem.

Based on what you've posted to date, I think the IAC is a likely suspect and you're on the right track.
 

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Discussion Starter #7
Yes it definitely seemed to...I know this may be a long shot and wishful thinking, but could the rough idle and faulty iac have caused the p0420?
 

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Mambeau / Admin
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Air induction issues can trigger conditions that might help throw a P0420 code, but I'm not sure air induction problems could directly trigger a P0420.

With the exception of a totally clogged up catalytic converter I can't think of a reason something wrong back at the cat would cause the no-start condition you described.
 
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