781-588-3968 bobtec@comcast.net

Fits most Bosch O2 sensors.

As most know VW and Audi’s have some weak converters and the old P0420/430 group of check engine lights that come with them.

There are few fixes out there. Does it need a new O2 sensor? Or is the wiring bad? Is the ECM working right? Or do you install a OEM converter for Big Bucks or find a good after market that still comes with a high cost?

With the Tom Thumb you can test everything in one test drive.

Some use an anti fouler to shut off the Check engine light – most of the time this never works right. It is added to the end of the rear O2 but adds like 1 1/2 inches to it, and sometimes hits the floor board.

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

Two things must happen when the engine tests its converters: first, it looks to see that the rear O2 does move when fuel trim is adjusted, and second, it looks to see that the rear O2 is slower than the front O2s movement by some kind of %.

So I built the Tom Thumb. It fits on the end of the Bosch O2 sensor like a chinese finger toy. You just push it on your rear O2 sensor hard, reinstall the O2 and test drive.

The Tom Thumb slips on the Rear O2 and into the converter. This is a test tool to make sure the ECM is in control.

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

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 it shows 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 the dimples help air flow around and inside 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 anything, and is made of hi temp steel.

The Tom Thumb is the first Catalytic Converter Test kit on the market of its kind it comes with everything you need.

Very simple to use just remove the rear O2 sensor slid on the Tom Thumb (Push it on Hard) on to the end of the Bosh O2.

We give you some furnace cement to help hold it to the O2 if you wish to smear it on the matting edge and some lube for the threads.

Reinstall the O2 and clear the codes with a scanner. Now just test drive on the high way at 55-65 MPH for a few miles.

Now retest the readiness for the Catalytic Converter  if it passed. The Tom Thumb test kit just tested the O2 sensor, the wiring from the O2 sensor to the ECM and tested the ECM itself.  The Catalytic Converter is BAD replace it.

I do have a new How to VIDEO:


I do not sell them over the internet any more, I install them at my shop here in Pembroke, MA.