Car Doctor Q&A: Why Is My Check Engine Light On?

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This week, Car Doctor John Paul fields a question about a Service Engine light illuminated on the dash of a Toyota Camry from the Clinton Era:

Q. I have a 1999 Toyota Camry and the “service engine” light came on. The repair shop told me it was the “evap” system but they couldn’t find anything wrong. They said the problem wasn’t on the fuel side. They reset the computer and all has been well for almost three weeks. This morning, the light came back on.

What exactly is the “evap” system and should I be concerned about the light? I love this old car, it runs great and has never disappointed me. I hate to get rid of it because of a check engine light no one can fix!

A. The “evap system” is shorthand for the evaporative emission control system, which prevents gasoline fumes from escaping into the atmosphere and causing additional air pollution. Since 1996 most cars have had an onboard test of this system to ensure it doesn’t leak.

Check Engine

Although the car’s performance will not change when the “check engine” light is illuminated, your car is causing air pollution. As a general rule the most common cause of an evaporative failure is a loose or faulty gas cap. With your year Toyota Camry the repair is generally replacing the evaporative VSV (vacuum switching valve) or purge solenoid. This repair will take less than an hour and the part is pretty easy to test.