FAQs

In the early 1980s, vehicle manufacturers began using electronics and on-board computers to control many of the engine control systems, such as fuel and ignition. Accordingly, they had to develop ways to diagnose problems generated by the new electronic hardware found under the hoods of those vehicles and, as a result, the first on-board diagnostics (OBD) systems were introduced. A more sophisticated on-board diagnostics system was eventually developed, and the second-generation on-board diagnostics system became required for most 1996 and newer passenger vehicles, light-duty trucks and similar vehicles.

The engines in today's vehicles are largely electronically controlled. Sensors and actuators detect the operation of specific components (e.g., the oxygen sensor) and activate others (e.g., the fuel injectors) to maintain optimal engine control. An on-board computer, sometimes referred to as a “powertrain control module” (PCM) or an “engine control module” (ECM), controls all of these systems. The on-board computer is capable of monitoring all of the sensors and actuators to determine whether they are working as intended. It can detect a malfunction or deterioration of the various sensors and actuators, usually well before the driver becomes aware of the problem using typical methods such as loss in vehicle performance or drivability. The sensors and actuators, along with the diagnostics software in the on-board computer, make up what is called “the OBD system.”

OBD is an emissions control system. Older emissions tests that collect or sample the exhaust produced by a vehicle identify vehicles that are already excessive polluters. However, OBD is a shift to pollution prevention. OBD can identify problem(s) with the emissions control system before the vehicle becomes an excessive polluter, allowing time to repair the vehicle before emissions increase. Left un-repaired, further damage can occur and emissions will increase. In most circumstances, the vehicle computer will detect a system problem before the driver notices a drivability problem. Furthermore, OBD can detect problems that may not be noticeable upon visual inspection because many component failures that impact emissions can be electrical or even chemical in nature. By detecting these emissions-related failures and alerting the driver to the need for potential repair, vehicles can be properly repaired before emissions become a problem.

When the OBD system determines that a problem exists, a corresponding “Diagnostic Trouble Code” (DTC) is stored in the computer's memory and a special lamp on the dashboard called a Malfunction Indicator Lamp/Light (MIL) is illuminated. The MIL is reserved for emissions problems only and cannot be used for other failures. Automobiles use a variety of warning lamps to notify drivers of different conditions. In the case of an emissions MIL, either the phrase “Service Engine Soon” or “Check Engine” is used or an engine symbol is displayed. This light, usually yellow in color, serves to inform the driver that a problem has been detected and vehicle service is needed.

When the vehicle is delivered to the repair shop, a service technician can quickly retrieve the stored diagnostic trouble codes (DTCs) from the computer memory of the vehicle. The stored DTCs will help the service technician diagnose and repair the problem. It is important to note that an illuminated MIL is intended to inform the driver of the need for service, NOT the need to immediately stop the vehicle. However, service should be sought as soon as possible. Drivers also may wish to consult a repair shop or their vehicle owner's manual for further guidance.

Under certain conditions, the MIL will blink or flash. This indicates a rather severe level of engine misfire. When this occurs, the driver should reduce speed and seek service as soon as possible.Severe engine misfire over only a short period of time can seriously damage emissions control system components, especially the catalytic converter, which is typically the most expensive to replace. Drivers should also consult their vehicle owner's manual for manufacturer specific information.

OBD systems are designed to automatically turn off the MIL if the conditions that caused a problem are no longer present. If the OBD system evaluates a component or system three (3) consecutive times (with some exceptions) and no longer detects the initial problem, the MIL will turn off automatically. As a result, drivers may see the MIL turn on and then turn off. For example, if the gas cap is not properly tightened after refueling, the OBD system can detect the vapor leak that exists from the cap not being completely tightened. If the gas cap is subsequently tightened, the MIL should be extinguished within a few days. This is not an indication of a faulty OBD system. In this example, the OBD system has properly diagnosed the problem and accordingly alerted the driver by illuminating the MIL.

Service technicians can manually turn off the MIL, but if the MIL is turned off manually without correction of the underlying problem that caused the initial MIL illumination, the OBD system will reset the readiness monitors (see “What are readiness monitors?” in the FAQ below), and begin the evaluation of the emissions control systems. If the OBD system has not completed the evaluation prior to the emissions test (see “How do readiness monitors affect the automotive repair industry?” in the FAQ below and “How can readiness monitors be set?” in the FAQ below), the vehicle will be rejected as “not ready”. If the OBD system completes the evaluation and detects the problem again, the MIL will re-illuminate, and the vehicle will fail the emissions test. Therefore, simply turning off the MIL prior to the emissions test, will NOT allow the vehicle to pass.

1996 and newer vehicles manufactured for sale in the U.S. include software designed to indicate when emissions control systems have a defect that may lead to elevated emissions. These systems include readiness monitors that are used to find out if emissions components have been evaluated. In other words, if all monitors are set to ready, the emissions components have been tested.

These monitors are included in the OBD system so the overall vehicle condition can be easily assessed electronically at an emissions testing station, using a data link connected to the vehicle. To ensure that the data provided to the testing station reflects the actual vehicle condition, the vehicle computer reports whether or not the data in the computer is current and valid. This is referred to as the “readiness” check. The vehicle will be rejected from testing if it fails the readiness check. A vehicle can have up to 12 monitors built into the OBD computer system. The most common monitors are:

• Continuous

  • Misfire

  • Fuel System

  • Comprehensive Components

• Non-Continuous

  • Oxygen Sensor

  • Heat Oxygen Sensor

  • Catalyst Efficiency

  • Evaporative Emissions System

  • EGR System

  • Secondary Air System

  • PCV System

In the past, technicians routinely cleared codes, either before and/or after performing repairs, as a routine part of the repair process. Clearing codes resets all monitors to a “not ready” status. Some monitors are easily run during normal driving; however, others can take a long time to run. This can cause problems if the vehicle has to return to a testing station for a retest because vehicles cannot be tested if a sufficient number of monitors have not run.

Remember, vehicles are rejected when more than two (2) non-continuous readiness monitors are “not ready” for 1996 to 2000 model year vehicles, or when more than one (1) non-continuous readiness monitor is “not ready” for 2001 and newer model year vehicles. Additionally, the catalyst monitor must be “ready” at the time of any retest after a vehicle failed an initial test for any catalyst code.

By clearing codes during the repair process, shops and technicians may find themselves in situations where the customer takes the vehicle back to an emissions station for a retest after repairs have been made, but the vehicle is rejected at the station because a sufficient number of monitors are not ready. Therefore, shops and technicians should seriously reconsider the practice of clearing codes on OBD equipped vehicles when that vehicle has to return for an emissions retest.

There are many reasons the readiness monitors could be set to “not ready”. One cause is routine maintenance. For instance, if the battery is disconnected for any reason, the monitors of most vehicles are reset. Also, a service technician may have to reset them as part of the repair process. In these cases, the vehicle must be driven to reset the monitors. Some manufacturers advertise driving procedures while others do not. The vehicle manufacturer or a qualified service technician is the best source for this information. Another cause is that there is a problem with the OBD system that prevents one (1) or more monitors from running. In these cases, a qualified technician must diagnose and repair the problem before the monitor will run. Before any of these monitors set to “ready”, the components must be operated in a specific manner designed by the manufacturer that checks the performance of that particular emissions control system. However, it is important to understand that setting a monitor to “ready” does not ensure that the system is defect-free. Completed readiness monitors only indicates that the applicable component or system has been checked.

Three monitors on all OBD-equipped vehicles have been designed to continuously check for system defects. Because of this, these monitors report as complete at all times and will not cause a vehicle to be rejected during a vehicle emissions test. These continuous monitors evaluate the following emissions control systems in the vehicle:

• Misfire

• Fuel System

• Comprehensive Components

In order to set readiness for non-continuous monitors, the vehicle must be prepared and driven in a specific manner. Monitors cannot be set using a scan tool, and the OBD system software must see the driving conditions required to run the monitor in order to complete the evaluation. These required driving conditions are referred to as “drive cycles” and define the preconditions and driving conditions necessary to set the readiness monitors.

Since many different readiness monitors may be present on a vehicle, drive cycles can be designed to set all monitors present with one (1) cycle, or an individual drive cycle can be targeted at specific monitors on the vehicle. It should be understood that both the catalyst and evaporative monitors require more driving than other monitor drive cycles.

Due to the complex interrelations between OBD system components and monitors, it is possible for one (1) defect to mask or “block” an additional defect(s). Because of this, it is possible to perform a valid repair on a vehicle only to have an additional defect identified by the readiness monitors shortly after the repair.

After a vehicle fails an OBD test where any of the fault codes are for catalyst efficiency, the catalyst monitor must be “ready” prior to a retest. Such vehicles presented for a retest with the catalyst monitor “not ready” will be rejected.

The intent of OBD systems is to ensure proper emissions system operation for a vehicle during its lifetime by monitoring emissions-related components and systems for malfunction and/or deterioration. An important aspect of OBD is its ability to notify the driver of a problem before the vehicle's emissions have increased significantly. If the vehicle is taken to a repair shop in a timely fashion, it can be properly repaired before any significant emissions increase occurs. OBD systems also provide automobile manufacturers with valuable feedback from their customers' vehicles that can be used to improve vehicle and emissions control system designs.

OBD systems are designed to alert drivers when something in the emissions control system begins to deteriorate or fail. Early diagnosis, followed by timely repair can often prevent more costly repairs on both emissions control systems and other vehicle systems that may affect vehicle performance, such as fuel economy. For example, a poorly performing spark plug can cause the engine to misfire, a condition sometimes unnoticed by the driver. This engine misfire can, in turn, quickly degrade the performance of the catalytic converter. With OBD detection of the engine misfire, the driver would be faced with a relatively inexpensive spark plug repair. However, without OBD detection, the driver could be faced with an expensive catalytic converter repair in addition to the spark plug repair.

Furthermore, manufacturers have increased incentives to build higher-quality vehicles with better performance, reduced emissions and more efficient powertrains to prevent problems that can lead to OBD detection. OBD systems also provide far more information than ever before to help auto technicians diagnose and properly repair vehicles during their first visit to the repair shop, saving time and money for consumers.

Federal law requires that the emissions control systems on 1995 and later model year vehicles be warranted for 2 years or 24,000 miles. Many automakers provide extended warranty coverage beyond what is currently required by federal law. Federal law also requires that the on-board computer and the catalytic converter on 1995 and later model year vehicles be warranted for 8 years or 80,000 miles.

Only qualified, trained technicians equipped with proper diagnostic and repair equipment should conduct OBD-related service. Vehicle owners should ask dealers and independent repair shops if their technicians have received proper training and have access to the necessary equipment to properly service OBD equipped vehicles. The Repair Shop Report (RSR) is an excellent resource to help select a qualified repair shop. A motorist can search for repair shops by city or zip code within a user-defined radius.