Dear Car Talk:

I have a 2004 Toyota Tacoma SR5 with about 85,000 miles. My check-engine light came on at about 75,000 miles, and it was a bad catalytic converter. The dealer said to let it go because they are so expensive to change. The light came on again as I was driving out of the dealership, but as any college student would do, I ignored it and put off the repair.

When I took the car to a different mechanic to have some other work done on it, I had the mechanic look at the catalytic converter to make sure the code wasn't being triggered by a bad O2 sensor. "Definitely the catalytic converter," he said, "but the problem is, there are two of them." He said that there is no way to tell which converter is bad, and that when I do decide to drop the cash, both will need to be replaced. Is that true? Is there any way at all to tell which is the bad and which is the good? The mechanic told me that if I was his college kid, and I wasn't experiencing any performance issues, he'd tell me to keep driving it. Thanks.

-- Ben
If you were my college kid, I'd tell you that you're polluting the air that the rest of us have to breathe. And that you have a civic responsibility to your friends and neighbors to fix this thing and stop doing that. 

Of course, if you were my kid, you'd then turn around and borrow the money from me, which would give me a lesson in keeping my nose out of other people's business. 

Anyway, there are two converters in this Tacoma. You must have the four-cylinder engine, because the six-cylinder engine has three converters! 

And it'll cost you $1,500 or so to replace both converters and both oxygen sensors with Toyota parts. But you can do it piecemeal. The front converter in this truck is the one that's monitored by the computer. That's the one that does most of the heavy lifting (i.e., the converting of polluting compounds to less-polluting compounds). So that's the one I start with. 

And while you can't buy a used converter, you can shop around and find companies that remanufacture old converters. They rebuild them, refresh the catalysts and sell them quite a bit cheaper than new converters -- about two-thirds the price, or even less.

We tend not to use them for most customers, because they don't always fit very well. And, in our experience, they don't last nearly as long as factory converters. But if you're short on funds, and you're trying to buy yourself a year or two for a few hundred bucks, that could be a good option for you.

And when that converter dies, you can see what kind of shape the truck and your bank account are in, and decide what to do. Maybe you'll have graduated by then and will be working on Wall Street, and you'll have converted this Tacoma into a Bentley Continental GT.

Good luck, Ben. But don't forget to consider your fellow man, and woman, when deciding what to do -- especially if you are headed to Wall Street.