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code analysis please! P0420
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XENOCIDE
Novice
Mar 15, 2010, 2:17 PM
Post #1 of 15
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code analysis please! P0420
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I have a 2002 VW GTI w/ about 100k miles
Ok. First off, I have looked up the 4 codes from the last service.
Bank1: O2S (Lambda) Correction Behind Catalyst: Control Limit Reached : P1176
Random Misfire Detected -Any Cylinder or Random/Multiple Cylinder Misfire Detected:P0300
Cylinder #1 - Misfire Detected or Cylinder 1 Misfire Detected : P0301
Cylinder #2 - Misfire Detected or Cylinder 2 Misfire Detected : P0302
They replaced the O2 sensor and the cylinder 1&2 coil packs
The CEL came on 2 days later so I bought a code reader for $30 (since each reading and resetting of the codes by my mechanic cost $42) and found this fault:
Converter - Low Efficiency or Catalyst System Efficiency Below Threshold (Bank 1) - P0420
Any thoughts?
(This post was edited by XENOCIDE on Mar 16, 2010, 6:53 AM)
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steve01832
Veteran
Mar 15, 2010, 5:29 PM
Post #2 of 15
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Re: code analysis please! P0420
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The low efficiency code is a pre-cat and post-cat comparison between the oxygen sensors. One thing I have found with VW and other German cars is that when an O2 sensor is replaced, it has to be a factory sensor. I have seen aftermarkets wreak havoc for some reason.
This code shouldn't give you a random misfire code however. You may want to contact the dealer and give them your VIN number. It is possible they may have an updated PCM flash that might be a fix.
Another thing that can give false misfire codes is a bad crank sensor.
Steve
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DanD
Veteran
/ Moderator
Mar 16, 2010, 5:28 AM
Post #4 of 15
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Re: code analysis please! P0420
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I agree with Steve about the OEM sensors on VW; on most vehicles actually.
The Tech that you’re dealing with should be able to confirm whether the O2 is functioning properly.
As for the new code P0420; if both O2 sensors (pre & post) are functioning properly; the misfires, may have burned the converter out?
It is easily confirmed, whether the converter is functioning properly, by the use of a gas analyzer.
Taking a pre converter reading of the gases; a post converter reading (tail pipe) and then using a math formula; you can determine the percentage of efficiency of the converter.
To get the pre reading; we usually drill a small test port in the exhaust pipe, in front of the converter; plugging it afterward with a self taping bolt.
Now mind you the vehicle must be running “normally” and supplying the converter with the proper, allowable amounts of CO, HC & O2 to the converter; if not then the above test is useless.
Here’s the formula.
Pre cat gas subtracted by post cat gas, divided by pre cat gas, multiplied by 100; will give you the percentage; anything below 70% efficient is considered weak.
Never thought I would be using calculators and math formulas, to diagnose and repair exhaust systems. LOL
One last thing; if this does turn out to be a burnt out converter; bite the bullet and buy an OE converter from the dealer. I do a lot of emission test fail diagnostics and I have yet found an after market cat that will run any better then about 70%. Put one of them on and your P0420 will likely come back to laugh at you.
Dan
Canadian "EH"
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XENOCIDE
Novice
Mar 16, 2010, 6:57 AM
Post #5 of 15
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Re: code analysis please! P0420
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I just want to clarify that after the the first repair concerning the first four faults listed the CEL did go out. It was not until a day or two later until the CEL came back on (I guess the time it took for me to drive one cycle after the repairs). It would seem very odd timing for the converter to go bad like that.
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DanD
Veteran
/ Moderator
Mar 16, 2010, 11:56 AM
Post #6 of 15
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Re: code analysis please! P0420
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Sorry but I didn’t mean to say that the converter had possibly gone bad after the first repairs.
The scenario I was suggesting was; that the misfiring cylinders allowed the converter to overheat due to raw fuel entering the converter and burning it out. The original post O2 was also not functioning properly and the computer could not pickup on the converters low efficiency.
Now that the misfire has been corrected and assuming that the replacement O2 is producing a proper signal; the computer has flagged the P0420.
In other words the first set of repairs may have been successful; to the point that the computer can now uncover a new fault or a fault that has been caused by the misfire.
Like I said in my first reply; after confirming that the replacement 02 can correctly respond to the oxygen content in the exhaust. You may need to have the converter tested for its efficiency with the use of a gas analyzer.
All of this is speculation, on my part; I’m not the one doing the testing. But I have seen good O2 replaced because of a bad cat and a perfectly good cat condemned because of a defective O2. Both the converter & the replacement O2 need to be tested to confirm who’s at fault.
Dan.
Canadian "EH"
(This post was edited by DanD on Mar 16, 2010, 12:02 PM)
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XENOCIDE
Novice
Mar 16, 2010, 12:12 PM
Post #7 of 15
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Re: code analysis please! P0420
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Thanks for that info!
So if the mechanic is telling me the converter is bad he must of performed a gas analyzer test, because that would be the only way to know for sure.
Also...this car just passed inspection. If I were to drive a car with a bad converter what would be the consequences?
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Hammer Time
Ultimate Carjunky
/ Moderator
Mar 16, 2010, 3:56 PM
Post #8 of 15
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Re: code analysis please! P0420
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When you see a PO420, it's pretty much always a bad converter. The criteria the reading have to meet in order to set that code are nearly impossible by any other means or failure. All the stars have to align correctly for that code to set. Mainly, both the front and the rear O/2 have to mirror each other. You can have a problem with either of the sensors and it won't cause them to mimic the other sensor and that's what it takes to set that code.
It's no problem to drive in the early stages because it just means the converter is not chemically doing what it should. It will cause you to fail emissions annd eventially, the converter will probably break up and cause a restriction so you don't want to ignore it indefinitely.
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We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.
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XENOCIDE
Novice
Mar 16, 2010, 5:31 PM
Post #9 of 15
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Re: code analysis please! P0420
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Ok. Just wondering though....if the converter is bad and the p0420 code didn't come up because of a bad o2 sensor, shouldn't I have failed emissions anyway?
And by saying I could drive with a bad converter for now....would it be crazy to think I could make it to next march's inspection?
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Hammer Time
Ultimate Carjunky
/ Moderator
Mar 16, 2010, 5:44 PM
Post #10 of 15
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Re: code analysis please! P0420
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Not necessarily, the emissions test just uses a snapshot of the emissions reading which could have been passing at the moment and the cars computer gets it's info over a longer period of time.
I couldn't tell you how long you can drive it. I wouldn't wait till the last minute though because it will take the car a while to go through all the drive cycles after the computer has been cleared.
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We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.
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DanD
Veteran
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Mar 17, 2010, 1:31 PM
Post #13 of 15
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Re: code analysis please! P0420
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Who in the hell said anything about this person doing his own diagnostics?
Not once during this thread did this person say that he was; but he was (I believe) trying to qualify the tech’s diagnosis; that is working on the vehicle.
I was explaining that there doesn’t have to be any guessing, as to whether the cat is functioning properly or not.
Sure go ahead and replace the cat; based solely on that code; you’d probably be right 75% of the time; but I don’t play the odds game with my customer’s vehicles.
Sorry for this Xenocide; I like confirming a diagnosis and have the equipment to do it; like most of the up to date shops should have. But there are some that feel comfortable with; the by guess by gosh approach and are quite arrogant about it.
Awaiting your reply HT; I’m 75% sure (just like the code diagnosis) there will be one.
You always seem to need the last word on any subject. LOL
You can rant all you want; because this will be my last reply to this thread.
Dan.
Canadian "EH"
(This post was edited by DanD on Mar 17, 2010, 2:44 PM)
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XENOCIDE
Novice
Mar 22, 2010, 2:18 PM
Post #15 of 15
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Re: code analysis please! P0420
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well I took my car into the shop today and they ran a readiness procedure on the converter twice. The converter passed both times. The tech said if the CEL came back on it would most likely mean I would need a new converter ($700 p&l).
At this point I'm blaming the new O2 sensor they put in. If the CEL happens to come back on and its the same code I'll drive the car till next inspection. If it fails I'll just trade it in at that time.
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