By John Polstra

Last month’s article touched briefly on the topic of the “check engine light,” or “malfunction indicator lamp,” what it means and what needs to be done when it comes on. In this installment, I will answer questions drivers ask most frequently when the check engine lamp on their dashboard illuminates.

Q: The light went on, but then went off. So is the problem fixed?

A: Not necessarily. If the problem is intermittent, the light may go off for a while but then come back on the next time the computer detects the problem. If this is the case, it’s time to have a trained technician diagnose the problem.

Q: The light is on, but the car is driving fine. Must just be a problem with the light, right?

A: Unfortunately, this kind of optimistic thinking is wrong. The computer is designed to turn the light on only if it is detecting a problem with the operation of the engine or transmission. Get it checked out before it becomes a bigger problem.

Q: My check engine light is flashing on and off. What does that mean?

A: Cars built after 1995 can detect when the engine is misfiring and alert the driver by turning on the check engine lamp. If the misfire is severe enough to be causing serious damage to the engine or catalytic converter, then the check engine lamp will start flashing to tell the driver that he or she should stop driving as soon as is safely possible and have the problem diagnosed.

Q: I had my vehicle repaired a few months ago to turn the light off, and now it’s on again. Why does it keep coming on? Does that mean it wasn’t fixed correctly the first time?

A: When the check engine light comes on, your vehicle’s onboard computer has detected a problem. Each type of problem is assigned a diagnostic trouble code, or DTC. A unique number distinguishes it from the hundreds of other DTCs. Determine if it is related to your previous problem by comparing the DTC with the one stored previously. If they match, it indicates a continuing problem; if the DTC is different, it indicates an unrelated problem.

Q: How do I find out which DTC was stored in my vehicle’s computer?

A: You should be able to ask your repair facility to provide this information. In order to perform his diagnosis, the technician, equipped with the proper scan tool, has to retrieve these codes from the vehicle’s computer anyway. There are scan tools available that allow consumers to determine which DTC was triggered.

Q: Can’t you just disconnect the check engine light?

A: Definitely not. Because the check engine lamp is part of the emission controls of the vehicle, tampering with it in any way is illegal. Besides, if the light isn’t functioning properly, how could you ever know if another problem arises?

Q: Can I still drive the car? Should I stop immediately?

A: Provided that the vehicle appears to be otherwise running normally, and the check engine light is not flashing, then it is usually OK to continue driving it until you can have a technician diagnose the problem.

Q: How much is this going to cost me?

A: Depends on exactly what the problem is. The first step, however, is to determine the cause of the problem. This may require diagnostic testing. Once your technician has diagnosed the problem, he can then advise you on a course of action to correct it and what the associated costs may be.

John Polstra operates European Auto Works, 239 W. Evelyn Ave., Mountain View, with Cliff Greenman and Ralph Foglein. For more information, e-mail c@ea-w.com.