Problems persist after catalytic converter theft

Dear Car Talk:

I live in Brooklyn, New York, and I had my catalytic converter stolen. My mechanic put in an after-market converter because there were no Toyota ones available. I guess they can’t keep up with all the converter thefts.

After I had the car towed to the mechanic, they replaced the converter, the “downstream oxygen sensor,” and the sleeve and clamp. They also did an oil change, air filter, cabin air filter, rodent droppings cleaning, and put a security shield on over the new converter to (hopefully) prevent it from being stolen again.

Here’s my problem. The car is now burning through fuel. I’m getting maybe 7 mpg. The pickup is sluggish. And when I press the gas pedal, the car roars and vibrates.

The mechanic doesn’t know what is wrong! Please help! — Jennifer

What’s wrong is that they only stole the converter and not the whole car, Jennifer.

There’s a slim chance that, coincidentally, something else happened to the car at the same time — like your timing belt jumped. But it’s much more likely that you have an obstruction in your exhaust system.

If you drove into my shop with an older car and described these symptoms, my first guess would be that you need a new catalytic converter. When catalytic converters get old, the internal parts break apart, and they, essentially, clog up. When they get clogged up, the exhaust can’t escape from the engine. And if exhaust can’t escape freely, fresh gasoline and air can’t come in, so you get no power and terrible mileage.

And while it’s unusual for a new converter to have an obstruction, that’s the first thing I’d check for.

The easiest way for your mechanic to check for that is to remove that aftermarket converter and drive the car. It’ll sound like a Gatling Gun. He’ll scare the heck out of every pizza delivery guy in Brooklyn when he’s testing it. But if you’ve got poor acceleration with the converter installed, and he’s able to zoom away once that converter is removed, that tells him where the obstruction is. It’s either the converter itself or further downstream from the converter.

If the car seems to be “fixed” without the converter, then your mechanic should get you another new converter. Maybe from a different after-market company. If the problem persists, then something is obstructing the exhaust pipe between the converter and the end of the tailpipe, and he can look for that.

What’s causing the obstruction? Who knows. It’s possible that the thieves did some additional damage when they stole your converter and somehow obstructed your exhaust system.

Maybe they were just converter thieves in training, Jennifer.

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