Do check engine lights ever turn off by themselves?

Dear Tom and 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? What do you think? – 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.

Windows won’t budge in the heat

Dear Car Talk:

When it is extremely hot outside, the windows of my '97 Pontiac Bonneville will not go down. They always work in the morning, but not in the afternoon heat. Anything and everything associated with the windows has been replaced. This affects all windows. My mechanic is at a loss. It appears to be an electrical issue. Where do we start in an attempt to fix it before the summer? Thanks. – Robin

RAY: I was going to suggest that you try cooling off the master window switch with a bag of ice, Robin. That way, even if it doesn't get the windows to open, you can put the ice bag in your lap and stay cool that way.

My real suggestion is that you make sure your mechanic has the car in his shop when it’s actually misbehaving. So on the next really hot day, take it to him and park it in the sun with the windows up, and tell him not to look at it until after lunch.

Once he has the car and the windows are not working, he’ll be able to do a bunch of very useful tests and narrow down the problem significantly.

He’ll be able to see how far the current is actually getting. Maybe it’s getting to the master switch on the driver’s door, and that switch is bad; maybe it’s not even getting as far as the master window switch.

He also can check any of the various ground connections that could cause the electric window motors to fail, or even run a temporary new ground to rule that out.

My guess, given that you’ve probably replaced all the obvious stuff, is that he’ll narrow it down to something called the Body Control Module, or BCM. The BCM is a small computer that controls things like the windows, power mirrors, power door locks and climate control. It sits behind the glove box, under the dash or just on the other side of the firewall. He might even be able to find you a used one at a junkyard for $50.

But having the ability to test everything, while the car is in failure mode, will lead him to the answer. Or, if it doesn’t, consider joining the Ice Bag of the Day Club, Robin. Good luck.