OBD II is a system that has been installed in most 1996 and later cars and light trucks, intended to inform the driver of problems in the components that control the engine and transmission. The primary reason OBD II was invented is to reduce smog emissions caused by malfunctions, but it is also valuable as an alert to the driver that something is wrong…something that can affect gas mileage and drivability, or actually cause further damage to the vehicle.
While you are driving your vehicle, its computer is constantly monitoring and running tests on the various sensors, actuators and electrical parts that make the car run properly. If a problem is detected, a Diagnostic Trouble Code (DTC) is set, and the computer turns on the “Check Engine” light. (The light may say “CHECK ENGINE”, “SERVICE ENGINE SOON” or merely be an icon that looks like an engine.)
Basically, drive the car. There are specific driving conditions that must be met in order to run various tests, but most of them can be met in normal driving. One thing that helps is to cruise at a steady speed for several minutes, so a ten-mile freeway drive in light traffic is a good bet. Keep a steady foot on the gas rather than speeding up and slowing unneccesarily.
Here are step-by-step instructions on how to perform a basic, yet very effective, Drive Cycle that will complete the readiness monitors for your vehicle's emissions control system. The Drive Cycle is one of the methods used by the powertrain control module (PCM) to determine whether an emissions system repair was properly performed.
A Drive Cycle is a special test drive that duplicates the scenario of a person starting her car and making a short freeway trip, as if she were driving to work. While the Drive Cycle test is going, the engine computer runs little tests or "readiness monitors" to see if the emissions system is working properly.
When a vehicle has an emissions system problem, it almost always triggers a Check Engine or Service Engine Soon Light. This signals that an emission system problem and fault code has been recorded in the powertrain control module (PCM). The problem indicated by the fault code must now be accurately diagnosed and repaired.
After the proper repair has been completed and the fault code cleared, the PCM will run a series of self-tests to determine whether or not the repair actually corrected the problem and if the various emissions systems are running properly. If they are, they can now properly minimize the emissions released into the atmosphere from the vehicle's operation.
This process was designed to prevent a vehicle from slipping through an emissions test with a known problem. Until 1996, a common tactic was to turn off the Check Engine Light by clearing the code just before an emissions test, without performing the proper repair. The Drive Cycle and Emissions Readiness Monitors have, for the most part, stopped this unethical tactic.