Ray Magliozzi, syndicated columnist

Dear Car Talk: I have a 2008 Toyota Prius with 95,000 miles on it. About 18 months ago, the catalytic converter was stolen, and I replaced it with an aftermarket converter for about $200.

All was well until about a month ago, when the check engine light came on (error code P0420). I have replaced the lower oxygen sensor and the gas cap, but the light stays on.

I still get over 50 mpg in city driving with no problems. I went to a dealer, and they want $2,700 for a converter replacement! Is it safe or wise to keep driving the car? — Karl

It’s safe, Karl.

Code P0420 is “cat efficiency,” which means “time to replace the catalytic converter … again.”

Why would it be time to replace the catalytic converter just 18 months after you bought your last one? Because you bought a really cheap one for $200. Cheap converters will skimp on the catalysts — the rhodium, palladium, and dealer-boat-payment-ium — the stuff that makes converters expensive.

You’re actually lucky that it lasted 18 months. We’ve put in a few of those really cheap converters, and sometimes the Check Engine light comes on within days.

The reason it may not be wise to keep driving is that your converter is no longer doing its job. That means all the tree-hugging points you amassed by driving a Prius are being wiped out by the air pollution you’re adding to your neighborhood. And if you live in a state where they do emissions inspections, you definitely won’t pass.

So, you have several choices. If you can get that same brand of converter that you got last time, you could try another one. Maybe that’ll give you another 18 months.

By my calculation, you can get 13 and a half of those $200 converters for the price of one Toyota converter. And if they each somehow last you 18 months, that’s about 20 years. Maybe you can even get one with a free, theft-deterrent bumper sticker: “Warning to Thieves: Cheap Converter Installed.”

Of course, the next cheap one may not last 18 months. That may have been an outlier. It’s hard to know.

So, you might also check out some independent repair shops that specialize in working on Toyotas. They may be able to steer you toward an aftermarket converter that costs less than the Toyota part but that they’ve had good, long-lasting success with.

And consider a converter shield, which doesn’t prevent theft but makes it harder, and might encourage a thief to move on to an easier mark.

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