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Removing catalytic converters in 2000 Mercury Grand Marquis

If i took off both catalytic converters would i do any serious harm to the engine?

Harm ? Don’t know . Legal ? No !!!

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If you take them off, the check engine light will never turn off. If you live in an area that does emissions checks, you will fail. You will be committing a federal crime by doing that and polluting the air we both breathe.

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why do you want to do this . . . ?

You are much more knowledgeable than me. I’m curious, aside from what you stated, which I totally agree with, would this not have an overall negative affect on performance?
Or does the O2 sensor after the catalytic converter only serve to monitor the cats efficiency?

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Last I knew they don’t affect performance. Maybe the newer model cars would. But this old duffer thinks if it ain’t broke, don’t fix it. And definitely don’t break it.

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The before-cat O2 sensors help control the air-fuel mixture so the ECU can determine how long to hold the injector open. That’s pretty well known.

The primary job of the O2 sensor after the cat is to confirm that the cat is getting its job done by measuring the amount of oxygen in the exhaust. I have heard that they can be used to bring the cats up to working temperature faster by allowing the engine to run rich. The ECU would then be sensing exactly when to stop running rich once the cats are hot enough to work.

In theory, you don’t need downstream O2 sensors for the car to run at all. Remove the cats, though, and they will determine the O2 content is wrong and set a CEL. I doubt you could see much of a difference in MPG’s or performance. But I’d like others to weigh in with their experiences with this.

I recently replaced one of my downstream sensors on my Mustang. The car determined the sensor was “lazy” and not signalling properly. I looked at the output with an OBD2 reader and sure, enough, it was not switching high-to-low-to-high as fast as the other sensor (dual cats, dual exhaust). Impressive bit of software for the car to determine that on its own! With a new O2 sensor, the car runs the same as it did before but no longer throws a CEL.

@Mustangman, I may be a little confused. I was under the impression that post cat sensors are supposed to have straight line voltage because the cat converts to carbon dioxide. If it doesn’t convert properly then you get a wave. Precats will show a wave from .1 to .9 volts, and are considered lazy when response time is slow. How can you see a lazy post cat sensor?

When I strip out the output from the rear O2 sensors versus time, at steady speeds they are essentially flat-line, as you say. I notice when I am deccelerating, off throttle, they go lean. Makes sense since the car shuts off fuel but some air is still coming in.

When I blip the throttle hard at idle, I can see the sensors react and go a bit rich. The ECU is adding a little extra fuel, like an accelerator pump on a carb, so they go rich for a bit. Same for a hard acceleration, the mixture goes rich to boost HP and the after-cat sensors respond by reading rich, too.

By comparing the 2 signals, the “lazy” sensor would lag the other one. Both lag the pre-cat sensors.

Thank you for that. I’ve never known to utilize that. Very helpful.

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