GMC Repair: 1996 GMC Truck Repair, autohelp, travelling back home

Question
Dear "Autohelp"

Here is my dilemma. About two weeks ago, I was travelling back home from Atlantic City. Just outside of Carlisle on the PA Turnpike, My truck suddenly lost power and would not restart. I was towed to a repair shop where the Ignition module was  replaced alog with the distributor cap & rotor. My vehicle was returned to me with the SES light on and because I was in a hurry to get home I did not have time to ask why it was on.

BTW, when it broke down, I did smell something electrical and the mechanic advised me that it was the ignition module.

Normally I was advised to disconnect the battery for about 30 minutes to let the computer reset itself so codes would clear.

I did this and it was ok for a while until I left for work and it came on again. Next item I thought was fouled spark plugs. I did replace them and #4 fouled which I replaced again according to AutoZone's code scanner. Once again I reset the computer and same result, the SES light came back on.

I had the codes scanned again and it showed a P0420 code. instead of a misfire.

This has been going for a while now. I downloaded info as to what may trigger that code from Actron's website so the easiest place to start was removing the air filter and drive home. Power seemed a bit improved but the SES light remained on. I felt some minimal hesitation as well

I did a visual inspection and found what may be and exhaust leak from the manifold at the flange.to the Y pipe (A black soot)and I looked at the Catalytic Converter there is a small dent in it at the bottom of the bashplate but no evidence of a black soot which I assume would indicate a leak.  

I did notice the sound of a leak which also may be the culprit.

I'm trying to pinpoint the exact trouble as I do not want to sink hundreds of dollars (Which I do not have) in a hit & miss scenario.

In short, the truck is still shooting a P0420 code. It's my guess that there are residual problems after the breakdown I had that obviuosly need addresses and I ma requesting your help.

Thanks
Mike Anderson  

Answer
ANY leak that is present at the exhaust flange will cause the oxygen sensor(s) to read the fuel/oxygen ratio in the exhaust incorrectly thus the most likely cause of your SES light being ON.

A hole in the catalytic converter can also cause said problem.

I would suspect that the catalytic converter is partially plugged up with carbon deposits and that you have oxygen sensor problems of which will need to be tested.

1. Fix exhaust leaks.
2. perform an exhaust BACK PRESSURE test to confirm restricted exhaust.
3. test/replace oxygen sensor(s).
4. replace catalytic converter.


I can send you the test procedures to follow for a code P0420.

You will need to contact me at:


onlineautohelp@msn.com


In order for me to send you the information necessary related to your problem, as this website is NOT mine and I am only a VOLUNTEER helping answer BASIC simple questions to those in need as this website does not allow pictures to be sent..

Please include all of the information, MAKE, YEAR, MODEL,  and all necessary other information that you previously provided as well as the information that I just gave you when you contact me.



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