Dear Doctor: I own a 1997 Toyota Avalon with 118,000 miles. The “check engine” light comes on constantly and I’ve had it fixed many times. Now I’m having the shop shut the light off, drive it for two days and then go for my smog check before the light comes back on. What would cause the problem? Linda
Dear Linda: The mechanic cannot just shut the light off and you then go for the smog test. There are many reasons for a “check engine” light to go on. In your case, it is a two-drive cycle fault. This means the computer has to see the same fault on the second-drive cycle. A drive cycle is when the car is driven under certain conditions for a certain amount of miles and highway speeds. Each car manufacturer has different driving conditions to set the computer monitors during the drive cycle.