I Want To Remove My Cats. Can?

[yendor]

Manufacturers install pre and post catalyst sensors to check the quality of the exhaust gases. The one before the CAT ensures that exhaust is in a reasonable range for the CAT to handle and is usually used to change AF ratios. The one after the CAT is used to monitor the final output of exhaust. Anything above a preset level and it tells the car that the CAT is kaput.

So when you remove the CAT, both pre and post sensors see the same amount of NO2 and CO levels and hence your car raises an alarm cos it thinks your CAT went kaput. There is a way to prevent that but it's a trade secret. Ask your friendly neighbourhood exhaust shop to solve this problem.

As for the M54 engines (which i love...ha hah..) The pre CATs are there to clean up the exhaust a little and to heat it up further before the main CATs do their thing. The only reason i can think of is because, here in asia, we do more city driving and at lower revs, the CATs dun heat up enough to be effective. Hence the need for the pre-CATs to get the enhaust up to temp.

Cheers

[louis]

Originally Posted by yendor

So when you remove the CAT, both pre and post sensors see the same amount of NO2 and CO levels and hence your car raises an alarm cos it thinks your CAT went kaput.

yendor, interesting bit about the trade secret. just to clarify, are you saying that oxygen sensors detect NO2 and CO levels? i always thought they were called O2 sensors because, erm well, they detect O2?

[yendor]

Yup. it only detects O2. Elevated levels of O2 means leaner mixtures and hence should correlate to lower CO levels. You are right to point this out.

As legislation is only concerned with CO and NO2 levels, manufacturers have no cheap alternative at hand to allow then to install these sensors in production cars. Hence they use O2 sensors and correlate the data.

The CATs function within a specified AFR in order to be able to clean the exhaust. It's supposed to convert NO2 to Nitrogen and Oxides hence at the post CAT sensor, it should see lots of O2. More than the pre CAT sensor..

Cheers

[louis]

well, i thought the opposite to be honest, ie post-cat reading of oxygen is higher than pre-cat; while a cat reduces NOx to N2 and O2, it also oxidizes CO to CO2, thus consuming oxygen.

[yendor]

hmmm, this is interesting. I was under the impression that the O2 atoms from the NO2 would be more than sufficient to convert the CO into CO2..

I think this calls for an experiment to perform actual measurements....

lemme think of a way.