View Full Version : P0420 CEL

08-03-2014, 08:02 PM
So my 01 1.8T QM threw this code after replacing CTS. From what I've read the code is thrown for a faulty cat, but I've also seen that it could be from bad O2 sensors, but I've never thrown a code for O2 sensors. From my understanding there are 2 O2 sensors? One in the front and the rear? If that's true, I'm going to replace O2 sensors first as that is the cheapest and less labor intensive fix. If that does not fix the problem I'll be needing new cat. I'm not tuned and do not have vagcom. What I'm really asking is, anyone have experience with this code, and what options are possible with a test pipe vs straight pipe if I have to replace the cat with no tune? Thanks in advance for the help, I always have luck with getting results from you all [:D]

08-03-2014, 08:40 PM
It would either be your cat converter or your secondary o2 sensor. Delete the DTC and see if it comes back. Start with the cheapest fix (free) first.

08-03-2014, 09:23 PM
The code 0420 means you gotta stop hotboxing man... not picking up enough o2

08-05-2014, 09:35 AM
I cleared the code and it came back on so next would be the secondary o2? I've tried searching for where it's located but I can't find it. I know there's one clear as day on the pipe from the turbo but I'm not so sure about the other one because there are 2 right?

08-05-2014, 10:30 AM
It's beyond the cat. further down the downpipe. it points inward towards the transmission.

08-05-2014, 11:01 AM
It's right after the cat. Honestly it's easier to remove the car and slide it around to get to the secondary o2 sensor to replace it. Drench all those bolts in PB Blaster for a few days.

08-06-2014, 03:44 AM
Last night the cel went off, (I didn't clear it again) so it comes on and off periodically so would that mean it's a good chance that it's just the o2 sensors? And if I replace one I might as well just replace both right?

08-06-2014, 05:50 AM
If it ain't broke don't fix it.

You can end up going down a very long rabbit hole if you just start replacing everything on these cars.

08-07-2014, 08:45 PM
[:|] im kinda new to actually doing the maintenance myself and truly loving my car lol so I'm learning here too. i didn't see how expensive a stupid sensor was so thats new to me. so ill start with one and see whats up. Thanks for the help [up]

08-07-2014, 08:55 PM
Do you have to smog/get your car checked for emissions? Its not a performance issue (unless your cat is clogged) just an annoying CEL. If you are tuned check your tuner if they can delete the sensor or if you're going to get it tuned soon get the sensor deleted. BTW to that person who said its after the cat...the sensor is actually on the cat but at the furthest end.

08-08-2014, 09:19 PM
So i never mentioned this and i can say i did see this also when i was searching...92 vs 87. I think it may be the gas. When I'm short money i throw 87 gas instead on 92, as it calls for 91 or better. I just filled up with 92 and it went up within 1 mile lol so hopefully it says off