GM OBDII Drive Cycle
Help in finding stubborn problems:
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, 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.
General Motors Driving Cycle
This will cause diagnostics to be run on all sytems. Have scanner connected while testing, using freeze frame. An interesting (and very dangerous - trust me) exercise is to use AutoTap or other scannerto watch the various monitors change from "incomplete" to "complete" as you go through the various sections of the drive cycle. Hard to look at the scanner and drive at the same time.
A complete driving cycle should perform diagnostics on all systems. A complete driving cycle can be done in under fifteen minutes.
To perform an OBDII Driving cycle do the following:
1) Cold Start. In order to be classified as a cold start the engine coolant temperature must be below 50°C (122°F) and within 6C (11°F) of the ambient air temperature at startup. Do not leave the key on prior to the cold start or the heated oxygen sensor diagnostic may not run.
2) Idle. The engine must be run for two and a half minutes with the air conditioner on and rear defroster on. The more electrical load you can apply the better. This will test the O2 heater, Passive Air, Purge "No Flow", Misfire and if closed loop is achieved, Fuel Trim.
3) Accelerate. Turn off the air conditioner and all the other loads and apply half throttle until 88km/hr (55mph) is reached. During this time the Misfire, Fuel Trim, and Purge Flow diagnostics will be performed.
4) Hold Steady Speed. Hold a steady speed of 88km/hr (55mph) for 3 minutes. During this time the O2 response, air Intrusive, EGR, Purge, Misfire, and Fuel Trim diagnostics will be performed.
5) Decelerate. Let off the accelerator pedal. Do not shift, touch the brake or clutch. It is important to let the vehicle coast along gradually slowing down to 32km/hr (20 mph). During this time the EGR, Purge and Fuel Trim diagnostics will be performed.
6) Accelerate. Accelerate at 3/4 throttle until 88-96 km/hr (55-60mph). This will perform the same diagnostics as in step 3.
[7) Hold Steady Speed. Hold a steady speed of 88km/hr (55mph) for five minutes. During this time, in addition to the diagnostics performed in step 4, the catalyst monitor diagnostics will be performed. If the catalyst is marginal or the battery has been disconnected, it may take longer.
Last edited by z28pete; 07-25-2011 at 06:19 PM.