I talked to a mechanic about the gutted cat idea. He said it might have an effect on the air/fuel ratio because the 02 sensor will be seeing higher carbons and adjust for it. You guys see a problem here or should I just drill them and not worry?
On an OBDII (94+) car, you *might* run into problems. I've heard of people getting a check engine light because of it, others haven't.
On OBDII cars there are four O2 sensors. Two before the precats, two after the main cat. If you gut the precats, the rear two O2 sensors will see a higher level of Hydrocarbons going through the exhaust. I wouldn't say it's bad, but the potential check engine light might piss you off.
*All* 3S models use the two O2 sensors located *before* the precats to set the A/F in closed loop mode. The O2 sensors located after the precats in some newer models are used *only* to test that the precat is working correctly. Gutting the precats can only affect how freely the exhaust flows (and therefore turbo spoolup because of the larger pressure differential across the turbo - a good thing) and will not affect the O2 measurements for A/F.
My advice - find another (better that is) mechanic.
I heard one guy on this forum say that he just left a plug in the bung and tied the O2 sensor up. He said that taking it out all together might leave you with a check engine light as it would probable notice that they are missing all together. If you just tie them up it would simply read the relatively clean atmosphere underneath the car and would think that the cats are doing one hell of a job.
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