. It's time to explain the OBDII drive cycle. This is an important topic, once we demystify the drive cycle, you'll understand why the check engine light doesn't necessarily turn on immediately when there's a fault, or turn back off immediately once the problem is repaired.

The drive cycle is the sequence of events that must happen for the various system test programs to run on your car's computer. The drive cycle involves a cold start, and a certain amount of driving time at a variety of different loads. More details on those cycles later.

When is the drive cycle important?

First, to understand why the check engine light takes a while to come on. Not only must the appropriate test monitor run, in many cases, it must run twice, detecting the failure each time. If the test runs successfully two more times, it can turn the light back off.

Second, remember from a few emails ago that these system monitors must be complete to pass OBDII based emissions testing (early model years often allow a couple exceptions). If you ever get sent home from an emissions test and told to come back after some driving, you can get the job done faster by understanding the drive cycles. Remember, you can use AutoTap or another scantool to check the drive cycle status.

Unfortunately, drive cycles vary slightly from manufacture to manufacture. We've documented the Ford and GM drive cycles. Since they're both about a page long, and can be a bit dry to read, I'm just going to link to our write-ups on our informational web site, www.obdii.com.

GM Drive Cycle http://www.obdii.com/drivecycle.html