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Old 10-18-2001, 12:20 PM
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Join Date: Aug 2001
Location: Santa Maria, CA
Posts: 49
O2 Sensor and Catalytic Converters

A number of members of this forum have talked about removing the "Pre-Cats" (called Primary Cats in the manual) from their exhaust systems to achieve better performance. I believe the theory is less exhaust restrictions.

My question is:

Given that the O2 sensor is usually placed AFTER the Pre-Cats, what will this do to the resulting fuel mixture (lean/rich)?

The O2 sensor will now see exhaust gasses that are much higher in CO, Oxides of nitrogen and unburned hydrocarbons. This will also result in a higher concentration of Oxygen than would have been seen post-catalyst.

I believe this will make the Lambda control system set an operating point that is richer.

One corroborating piece of evidence is that one of the forum members burned his secondary catalytic converter out after removal of the pre-cats.

Thanks for any of your expert opinion.

Tom Elerding
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Old 10-18-2001, 01:18 PM
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Join Date: Oct 2001
Location: Canada
Posts: 298
I agree with you, on my Ford Explorer, there are down stream O2 sensors and up stream O2 sensors.

On my Ford, down stream O2 sensor locates between Cat and Muffler, someone did try to remove their Cats but it caused down stream O2 sensor reading going out of tolerance, and triggered check engine light.

However, on My ford, down stream O2 sensor is used only to monitor Cat performance, it doesn't involve in engine management, so it doesn't hurt the truck itself (but our environment). The up stream (pre-CAT) O2 sensor does involve in engine management.

If MB only has one downstream O2 sensor, it must be involved in engine management, so I think, it can cause air/gas ratio problem.
99 BMW 540i 6-speed 110K Km
03 SAAB 9-5 wagon 80K Km
92 400E (Sold) 245K km
Still missing the days with the Benz, it kept me busy.
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