Reader Question: Leaving Off the Cats?

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Here’s the latest reader question, along with my reply!

Todd asks: I read your recent article about people having the catalytic convertors cut off their cars by thieves. I’ve had the same happen to me and am reluctant to spend the money on a new converter because what’s to stop another thief from stealing, it too? But my question for you is – will running the car without a catalytic converter negatively affect anything?

My reply: Well, it will trigger the yellow “check engine” light in the dashboard to come on – and stay on – which is only a problem if you need to get a state safety or emissions test to renew your vehicle’s registration. And if that’s the case, the inspector is going to notice the absent cat, regardless. But there’s no negative effect of the light being on. Unless it just annoys you.

The car will be louder – because the cat has a muffling function as well an emissions function. This may or may not be a concern for you.

It may run better – and might get better gas mileage – because the cat is restrictive to the free flow of exhaust; an older/high-miles cat will be more restrictive due to the accumulation of carbon on the lattice/honeycomb surfaces within the cat. But it may also run worse, if there’s not enough backpressure; or because the computer alters the A/F ratio and so on because of the signals it’s getting indicating the cat is gone.

Some people get around some of this by wiring a resistor of the right value into the oxygen sensor circuit; this “fools” the computer into thinking the car’s cat is intact and operational.

You should be able to tell us more about what actually happens – after you run around for a few weeks, sans cat. Please keep us posted!

. . .

Got a question about cars, bikes or anything else? Click on the “ask Eric” link and send ’em in! Or email me at if the @!** “ask Eric” button doesn’t work!

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  1. Test pipe and ecu tune will capitalize on the added power and address the CEL

    Jersey doesn’t seem to care, sorry for the people who refuse to leave communist California though

  2. My family spent four years in Europe in the mid-’80s, courtesy of Uncle Sam. Our first stop was England, where unleaded gasoline was not available. We had a Chevette shipped over so we had something to get around in that wasn’t too large (unlike the ’73 Cutlass that was pickled in a garage in Pennsylvania), but before that happened, the catalytic converter had to be removed.

    In addition to it being a touch louder, it was also a bit more prone to backfiring. There was a steep hill just up the road from where we lived that went up to the main drag (the “high street,” as they call it). Backing off the throttle near the top of the hill (where there was an intersection) would nearly always elicit some popping noises.

    (A couple years later, we moved to Germany, where “bleifrei” was just starting to hit the market…at a lower price than leaded, to incentivize people to buy it. Contrast that with the higher prices we paid for unleaded because our all-knowing government decreed that we must buy it for our newer cars. That said, the catalytic converter wasn’t reinstalled until we returned to the States a couple years after that. A few months after our return, I started driving, and that Chevette was my first car.)

    • Funny, I thought I was the only one who took a Shove-it (Chevette) to Europe in the 1980s. Mine accompanied me to Germany for a couple of years. My dad removed the cat and put an appropriately sized pipe in the gap. I didn’t have the same problems you did.

      Since I live in a state that doesn’t require emissions or “saaaaaafety” inspection, I’d run without a cat now if mine was ever stolen. And I wouldn’t be worrying about the noise, either. I’ve noticed other cars around me now that are pretty noisy, so I guess it’s not a big deal.


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