DEAR RAY: I’m the original owner of a 2002 Toyota RAV4. It has about 135,000 miles on it. It’s the best car I’ve ever owned.

A few months ago, my check engine light went on. The car was running fine, but after two weeks of staring at the light, I took it to a mechanic. He told me the sensor for the catalytic converter is bad (about $400), and the catalytic converter also might be bad (another $600).

He told me converters usually last about 130,000 miles. It was not my fault; they just need to be replaced after around 130,000, give or take.

The car was running fine, 20 mpg as always. So I decided to wait till I got my income tax refund.

Three weeks later, after I changed my oil, I noticed that the check engine light was off.

Did the light burn out? No, because when I turn the key to start the car, I see it light up.

Is my catalytic converter sensor still bad? Do check engine lights turn off after a while? Or was the mechanic just looking for a $1,000 job?  – Russell

RAY: I wouldn’t go blowing that income tax refund on the director’s cut of “Bikini Car Wash II” just yet, Russell.

I don’t think the oil change had anything to do with it. I think what’s happening is that your oxygen sensor or the catalytic converter itself is on the verge of failing.

A check engine light will shut itself off if the condition that caused it is remedied. So, if your converter is marginal, and you did a lot of stop-and-go driving, which creates high demand for the converter, that may have turned on the check engine light.

If you then did a bunch of highway driving before changing the oil, the specs may have dropped back into the normal range, causing the light to go off.

My prediction is that the light is going to come back on again, if it hasn’t already by the time you’re reading this. And, at some point soon, it’ll stay on – and then you won’t be able to pass inspection.

So when the light comes back on, take the car, and the tax refund, back to your mechanic and have him try an oxygen sensor.

At 135,000 miles, it’s very likely that you need one. And if you’re lucky, you won’t need to replace the converter itself until after next tax season.

Got a question about cars? E-mail Car Talk’s Ray Maggliozzi by visiting the Car Talk website, www.cartalk.com.