How to fix Bad Cat codes P 0420 on VW and Audi Bosh O2 sensors

As most know VW and Audi,s have some weak converters  and the old P0420 group of check engine lights that come with them.

There are few fixes out there install a OEM converter Big Bucks or find a good after market ( I do have one brand I have good luck with ) still comes with a  high cost.

Some use a anti fouler  that is added to the end of the rear O2 but adds like 1 1/2 inches to it. But some times hits the floor board .

What they are trying to do is slow down the reading of the rear O2 so it do,s not match the reading of the front O2 sensor that is used for fuel trim.

Two thing must happen when the engine tests its converters first it looks to see that the rear O2 do,s move when fuel  trim is adjusted the next test is

it looks to see that the rear O2 is slower than the front O2s movement  by some kind of %

So I build the Tom Thumb it  fits on the end of the bosh O2 sensor like a chine,s finger toy it is adjustable on the small hole at the end you just drill it out to adjust it.

The Tom Thumb slips on the Rear O2 and into the converter so no one could ever tell the converter is a bit weak. This is a test tool to make sure the ECM is in control.

Here are some pictures of the Tom Thumb P0420 fix and the Vag Com scanner screens showing it working.

Just click on pictures to enlarge

The stock O2 location using the Tom Thumb

What it looks like with a useless anti fouler

This a bit hard to see but what its show is the bottom line is the front O2 swing for fuel trim.

The yellow line is the rear O2 swinging very close to the same rate of the front O2 at 700 RPM

This converter is ready to fail a P0420.

The same converter using the Tom Thumb almost like new .

This is the Rear O2 in the new converter green line and the rear O2 yellow using the Tom Thumb in the old converter at 700 RPM.

The Tom Thumb is build in a way so th dimples help air flow around and in side the new controlled chamber here  are some close ups.


Some have asked will it fall off NO it is a press fit . It can be pulled off if you must with some force and use on some other O2 sensor.

If it did fall off it is in the back of the converter so it will never hurt any thing made of hi temp steel too.

I am selling them on Ebay or just email me

I do have a new How to VID

Bob Mann


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