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Diagnosing P0422, with ECU live data logging

3510 Views 7 Replies 4 Participants Last post by  ldestrada5
Hello everyone,

I am in the process of diagnosing some problems with my 1998 Beetle 2.0L (AEG), stick transmission, 173k miles on it.

Background info

It's had the CEL on for a while now (maybe more than 2 years). No noticeable change in performance (maybe just drop in mpg, not too much power before 2k rpm, and it used to buck before 2k rpm as well). Anyway, I have fixed some things on it, including the secondary air pump, timing belt/water pump, thermostat and coolant exchange. I also did a BG109 treatment, followed by oil change and BG MOA additive. After this maintenance the bucking was gone (probably because of the secondary air pump being fixed).

It had 11 codes that I pulled out and logged back in February. I cleared the CEL after all the maintenance done above and drove it for a while, then the CEL came back on with only 2 codes: P0103 - MAF Signal too high, P0112 - IAT Signal too low. In July I looked at the MAF sensor connector on the car side and saw that the terminal for the IAT sensor (cable #5) was all bent (it did not have the protective plastic thing holding all the terminals together). So I figured the problem was the MAF sensor connector on the car side. I bought one from a junkyard and changed it (cut the cables and spliced them back together using the VW butt connectors with the glue lining). I then cleared the CEL and drove it. CEL came back on fairly quickly with the same 2 codes (measuring blocks showed IAT constant at 19.5 C in the middle of July, so I assumed it was not working). I changed the MAF sensor at this point and cleared the CEL. Drove it and no codes for a while, but then only 1 code came back after that: P0422 (16806 - Main Catalyst; Bank 1: Efficiency Below Threshold). I have cleared it once and it came back on after about 30-35 miles of driving. When I look at the readiness monitors, the last one to complete is the catalyst. The CEL (and P0422 code) comes back on right after the catalyst readiness monitor has completed.

I have done some research regarding this code and it seems to be either the O2 sensors or the Cat. The Cat has been changed once before with a universal one that was just cut and welded in place (this was ~5 years ago and it does have the O2 sensor bung in there). I have also logged ECU data from my drive to work and back, looking at RPMs, Speed, Fuel Trims and both O2 sensors (I have put some representative graphs at the end of the post from logged data at constant ~65 mph and ~3200 RPM over a period of time; using cruise control). If I am not mistaken, both sensors are working. I say this because they both can reach voltages up to 0.9 (front) and 0.8 (back). Additionally, the front O2 sensor (#1) cycles back and forth indicating lean/rich conditions (which is also seen in the Fuel Trim cycling to correct for this conditions). However, I think that the Cat is the non-working part because the second O2 sensor (#2), also cycles back and forth indicating lean/rich conditions even after the Cat.

I also did the temperature test of the Cat inlet and outlet. The temps are 233-241 F at the inlet (front of the car), 278-288 F at the middle of the Cat, and 292-299 F at the outlet (back of the car). I took this temps with an IR thermometer with laser guide. I looked at the video from ETCG for diagnosing a bad Cat and I got from it that a bad Cat would be cooler at the outlet than the inlet. And a good one should be about 200F higher at the outlet. So maybe my Cat is not all the way gone, but getting there; hence the code.

Now to the actual questions:

1. Is this the right conclusion (Cat needs to be replaced) based on the diagnosis that I have done?
2. Do I need to do any further testing before changing anything?
3. Is my assumption that the O2 sensors are working correct? Based on the voltage range seen by the ECU data.
4. And is my assumption about the temperature test correct?
5. Could my Cat not working efficiently anymore be due to the issues I had before with the secondary air pump and MAF/IAT sensors? I think this was making it run too rich and eventually ruined the Cat.

All of the help and comments that you guys can provide will be greatly appreciated. And please let me know if anything is not clear or if you need any more information that I should get or things to look at. And sorry for the long post, but I wanted to get all the info I had relevant to the code out there. All of the threads that I read in the forums here regarding P0422 never posted a solution to it other than the VW extended warranty (which does not apply to me at this point).

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Spacers work good. You will never see that code again!
Meow. :)

Sent from AutoGuide.com App
Spacers work good. You will never see that code again!
Do you mean spacers for the O2 sensor after the Cat (#2)? So that means that my Cat is gone, correct?
I agree with everything you've said. Answers are all yes, except #2, you don't need to do any more diagnosis. Wow, it would be easy if everyone did as thorough a diagnosis job as you!

The aftermarket cats don't last nearly as long as the OEM ones because they don't contain nearly as much of the rare-earth ingredients (you get what you pay for and all). 5 years might be typical for a life. The unhealthy MAF certainly could have contributed. That is why it is bad to go on driving for a long time with the CEL on: you may know you can ignore the original reason it came on but if something else goes bad you have no way of knowing and that can do the harm.

The rear O2 sensor should always see a very lean condition if the cat is working because the cat should use all the rest of the Oxygen burning up the remaining hydrocarbons. The fact that the cycling to higher voltages is present in the rear O2 sensor signal means both that the sensor is good and the cat is not doing its job.
Thanks for your reply. I shall order the Cat and get it installed. I'll post after I do that to confirm if the CEL stays off.

Wow, it would be easy if everyone did as thorough a diagnosis job as you!
I figured I should not waste people's time. So I gathered as much info as I could to narrow down the possibilities.

Again, thanks! :)
I ordered a universal catalytic converter and got it installed Friday. Drove the car around for about 50 miles before all the readiness tests completed and after the last (Catalytic) readiness monitor completed... No CEL! Woohoo! I've attached data graphs with how the O2 Sensor2 looks now with a working Cat. A lot steadier than before.

Thank you for your confirmation about the Cat diganosis red99! Appreciate the help.

Long live the ORG!

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Hey, would you mind; posting info about the universal cat that you bought? That way; people can purchase the same part with (hopefully); the same positive results as you! Congrats, on the very good systematic approach to the troubleshooting and repair of the classic P0422 code! Good job! :)
Definitely! The Cat that I bought is from Eastern Catalytic, Part #83704. I ended up buying mine from Amazon because that was the best price I found, and got it in about 4 days (ordered Sunday Aug 10th, delivered Thursday Aug 14th). Installed it the next day after receiving it (Friday Aug 15th), right before going to work.

I did the recommended procedure after installation: 1. Let engine warm up for 5min at idle. 2. Raise RPMs to 2500 and hold for 2min. 3. Let engine cool down. 4. Road test it.

It ended up working nicely for me because my work is about 1 mile from the muffler shop, so I got it installed, did the procedure. Drove it nice and slow to work (keeping RPMs below 1500 all the way), parked it all day (10:30am to 5:15pm) to let engine cool down and then took it home. Also, they had to install it a bit tilted to allow proper sitting of the second O2 sensor (see pictures below). However, that doesn't alter the function of the Cat as far as I know.

Hope this helps somebody with the same code or a suspected Cat problem.

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