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