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.)
OK, my monitors are incomplete…how do I complete them?
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.
What Is the Purpose of a Drive Cycle?
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.
Promote current deals
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.
Step 1: How To Prepare Your Vehicle
Step One: How to Prepare Your Vehicle
Have the fuel tank between 30 and 70 percent full. Some systems, especially the EVAP system, need to have a specific level of fuel in order for the tests to be trusted. If the fuel tank is near empty or completely full, many of the basic tests will not run at all.
The vehicle must also have a good alternator and a strong battery. If you have to occasionally jump-start your vehicle, all of the memory from the powertrain control module (PCM) is erased, which includes the data that accurately tracks the results from various stages of the Drive Cycle. Also, if the battery is weak or undercharged, some of the most important tests will never run.
The vehicle must sit overnight, or for at least eight hours, in an environment that is less than 90° F. The engine temperature needs to match the air temperature in order to establish an accurate baseline for the testing. If the outside temperature is over 90° F, the fuel is too volatile and the EVAP system won’t even try to run its tests, though some of the other emissions systems may run their tests.
The keys must be out of the ignition and all of the doors must be closed while the vehicle sits over night because many of the onboard computers “boot up” when the keys are in the ignition. Also, many of the onboard computers still run until all of the doors are closed after the vehicle is shut off and the keys are removed.
Step 2: The Cold Start
Step Two: The Cold Start
Start the vehicle and let it idle for two to three minutes in Park or Neutral. While it is idling, turn on the head lights, heater/defroster, and rear defroster for a three to five minute warm-up phase. Let the idle speed settle down to near the normal speed.
Next, put the vehicle in gear and drive through city streets at about 25 mph. Go up to about 35 to 40 mph a few times before slowing down to stop. Don’t roll through the stop; be sure the car is really stopped, just like you learned in driving school. Accelerate from each stop in a normal fashion—not overly conservative, but not like you are competing in a drag race either.
Step 3: A Short Freeway Trip
Step Three: A Short Freeway Trip
After the vehicle has been cold started and driven for a few miles on city streets, the next step is to take it on a short freeway trip.
Enter the freeway on-ramp and allow enough room with respect to other vehicles so that you can do a 1/2 to 3/4 throttle acceleration up to freeway speed.
When you have accelerated up to around 60 mph and have safely merged into the flow of traffic, stay in the slow lane and maintain a steady speed of 55 to 60 mph for a minimum of five miles. Please use the cruise control to help you maintain speed.
Find a nice, long off ramp to exit from the freeway. As you exit, take your foot off of the accelerator and let the vehicle coast down until it stops under its own power as you complete your exit from the freeway. Do not use the foot brake and do not shift gears until the very end of this “coast down” phase.
Step 4: More City Driving
Step Four: More City Driving
After you have completed the freeway trip, drive through the city streets for a repeat of the second part of Step Two.
Go up to about 35 to 40 mph a few times and then maintain a city speed of 25 mph before slowing down to stop. Again, don’t roll through the stop and make sure to accelerate normally.
Pull in to a parking place and let the engine idle for one to two minutes and then shut it off.
Step 5: Have Your Readiness Monitors Checked and Verified
Step Five: Have your Readiness Monitors Checked and Verified
Drive your vehicle to your regular shop and have them re-check your readiness monitors, present codes, and pending codes. They should do this as a courtesy and for free.
If all of your monitors are “ready” and there are no present or pending codes, then your vehicle has been properly repaired and is ready for an emissions inspection and for normal driving.
If your monitors are not ready, please Visit our shop for diagnostic. information.