Jim Grant's Tech Tips
'97 Jeep Grand Cherokee, Check Engine Light Folklore or?
Q: I was curious to know whether you could tell me how to turn off my "Check Engine" light? I own a 97 Jeep Grand Cherokee Laredo, 4.0 L. An auto parts store guy read the code for me, and said he thought it would turn off after about 15 or so "drive cycles". Any truth to that?
- Marcus G.
A: The Check Engine will turn off by itself if the problem is no longer present after about 3 drive cycles. Each time your vehicle is started and driven the computers software commands a wide variety of tests to be performed and monitors various sensors while the vehicle is being driven. A drive cycle is when the vehicle/engine sets long enough for the coolant to drop below a specified temperature. Then the engine is started and the vehicle is driven at different speeds and allowed to reach full operating temperature. The reason for this? The computer not only tests the system, it also monitors for range of operation. Certain tests can only be performed when the engine is cold while others are performed when the engine is hot. Each time the engine is started the computer performs over a hundred checks and tests of your vehicles emissions system, which includes the automatic transmission too. If your problem has been corrected, generally the Check Engine light will turn off within a day or two of normal driving. If the light does not turn off, there is a system problem and diagnosis is needed.