Though modern vehicles are cleaner due to newer engine technology and emission control components, motor vehicles are still the largest source of toxic and smog forming air pollutants in the US. Despite enhancements to motor vehicles, emissions are only low when the emission control systems are working properly. When an engine is not running as efficiently as possible, performance is lost, fuel is wasted and air emissions increase. On Board Diagnostics (OBD) can detect problems that may not be noticeable upon visual inspection because many component failures can be electrical or even chemical in nature. By detecting component deterioration and/or failures, and alerting the driver to the need for potential repair, vehicles will be properly serviced before emissions become a problem and expensive repairs are necessary.

What is OBD and how does it work?

On board diagnostic (OBD) systems were developed by vehicle manufacturers and the federal government to help technicians diagnose and service the computerized engine management systems of modern vehicles. Regardless of make, 1996 and newer vehicles use the same computer “language” and monitor the same components using the same criteria for evaluating the power-train systems. OBD can detect a malfunction or deterioration of certain components before the driver realizes there is a problem.

How does the driver know there is a problem?

When the OBD system determines that a problem exists, a corresponding “Diagnostic Trouble Code” is stored in the computer memory. If the OBD system detects a problem that may cause the vehicle emissions to exceed federal standards, then the Malfunction Indicator Lamp (MIL) is illuminated.

The MIL is a dashboard light indicating “Check Engine” or “Service Engine Soon” that informs the driver that a problem has been detected and vehicle service is needed. By law, this dashboard light can only be used to indicate an actual emission problem. It cannot be used for example, as a reminder for regularly scheduled maintenance. At a repair shop, a service technician can retrieve the stored diagnostic trouble codes from the computer memory using a computer “Scan Tool." By using this information, the technician can identify the problem and make the proper repair.

What do "readiness" monitors have to do with OBD?

The OBD system is made up of monitors. An OBD monitor is a test or series of tests used to determine operational status of an emission control device or system. Most vehicles have five to seven monitors, but some have up to 11. These self-diagnostic tests are performed during normal driving routines. When the test(s) are completed, the monitor(s) will set to "ready," meaning that the OBD system is ready to relay information. However, at times, monitors may not be "ready" to relay information and therefore cannot be inspected until the monitors are ready. A vehicle's monitors may not be set to "ready" because they have been cleared either after repairs were made or the battery has been disconnected or loses power. When these things happen, the vehicle must be driven long enough and far enough to complete a full diagnostic drive cycle; this is different for different vehicles and can take several days. A vehicle cannot be inspected until the monitors are set to "ready."

How do I ensure my vehicle’s monitors are ready?

Some self-diagnostic tests will not occur until you have driven the car at highway speeds for a certain length of time. Others will not occur until you have driven the vehicle the required number of “key cycles” of warming up to normal engine temperature and then cooling back down. Automobile manufacturers have various strategies for resetting the vehicle’s monitors. For this reason, we cannot give you a simple list of instructions. Drive cycles are a combination of highway driving, stop and go driving, idling, and for some vehicles an overnight cool-down period. You can check with your vehicle’s manufacturer for specific drive cycles. Ask your mechanic to make sure that your vehicle’s monitors are complete before bringing the vehicle back to a station for a retest. You should be confident that all monitors are complete before returning for a retest, otherwise your vehicle may reject due to incomplete monitor status. If the monitors are not ready, they have not run their diagnostic check of the vehicle’s system and the OBD test cannot be completed.

Check your warranty: Emissions-related repairs may be covered

Federal law requires that the emission control systems on 1996 and newer model year vehicles be warranted for a minimum of two years or 24,000 miles. Warranty coverage for the on board computer and catalytic converter (only) is extended to eight years or 80,000 miles for these same vehicles. Many auto makers provide extended warranty coverage beyond that required by law. So, depending on the model year and mileage of your vehicle, emission system repairs may be covered by the manufacturer.