Page updated on 04-22-2018

P0420 Engine Light 02 Jetta

07-18-2005, 07:14 AM
I have an 02 Jetta with the 2.0 engine. I have 69000 miles on it 30000 put on this year.

The engine light came on a last week on my way to work. I disconnected the battery to reset the light and assumed because the car is running fine that it was because of the humidity.

A couple days later it came on again so I pulled the codes and got:

Catalyst Efficiency
Below Threshold
(Bank 1)

I thinks it is one of 3 things

1) O2 sensor
2) MAF sensor
3) Bad gas

I ran the gas nearly empty and filled up at a reliable station and the light is still on -I am going to clear it tonight and see if it remains off.

If it comes back I will probably clean the MAF, and if is comes back after that I will replace the O2 sensor.

I was just wondering if anyone had a similar issue or has better ideas...

07-20-2005, 08:08 PM
more likely a bad cat than any of those other ones.

07-21-2005, 10:07 AM
Bad cat? That would really suck!

I ran the car near empty a few times and filled it up with good stuff -not high octane just good 87. I cleared the code and it has been off for about a day.

Hope its not the cat...

07-24-2005, 12:46 PM
Cat is covered under warranty 8yr 80k miles

07-25-2005, 07:04 AM
How would I know if it is the cat? Should I bring it in?

07-27-2005, 06:32 AM
Talked to a VW dealer -they said 7-70 for a warranty on the Cat so I'm SOL.

On the other hand I had the oil changed by a different VW place a few months ago -they overfilled it by at least a quart -I ran it like that for 2 days (260miles) before I could get back to the dealership to yell at them and make them redo it. It says right in the manual, if you overfill the oil it will go out the crank breather and right into your Cat causing damage. I will call them to complain and see if it will get me anywhere...

07-27-2005, 07:37 PM
Actually overfilling causes misfires. Misfires eat up a cats internal parts.

Cat should 8yr 80k, check your owners manual.

07-28-2005, 06:23 AM
No luck -I called 2 dealers and both said they cannot help me, both say the cutoff is 70K. The cat is so expensive I may as well get a cat back racing system. If the cat is bad I think I'm going to clean it with a few little tricks or hollow that puppy out if cleaning fails.

Is this code coming from the O2 sensor or is there a sensor right in or after the cat?

07-28-2005, 09:09 AM
Unless you live in california you can pick up a new cat for less than $400. If you live in CA, you can find a recertified salvage for about $400. sells aftermarket cats, 1-800-VW-PARTS sells salvage parts. I just bought a reconditioned cat... which brings me to my next question...

Is it possible to trade out the cat myself? I live in an apartment building and only have a small floor jack to use in my carport, but a full set of tools. What is involved in the job??

07-28-2005, 10:00 AM
Yeah I found an aftermartket replacement for 261 at

The replacement shouldn't be too hard but I noticed that the cat is a large section of exhaust rather than just being a small section. I'm thinking of cutting the cat section out, cleaning it myself and then clamping it in back in place. I am going to do mine on ramps.

The car should pass emmissions without a cat...

Doug Tatham
07-29-2005, 10:20 AM
The reason the Cats have such a good warrenty is that they don't usually go out.
There are a couple very inexpensive possiblities, both have given me the same error. Rember that fuel mix problems will lead to emmission problems that are mistaken for a bad cat.
1. Bad air filter. This can cause an improper mixture and an overy rich mix can lead to a cat error.
2. Fuel pump (not so inexpensive). Check you fuel pressure.
I have found that the cats tend to last for the life of the car unless your mixing coal with lots of sulfur with your gas. From what I've seen, corrosion tends to be the number one killer of cats.

08-01-2005, 06:33 AM
Yeah, the cat in function is very simple and should last the life of the car. Unless the catalyst metals inside become covered or clogged.

I drove for a week with the engine light on and then it just went off. Its been off for a couple days -I'm just going to leave it alone.

Add your comment to this topic!