Understanding On-Board Diagnostics (OBD)

General Information

The purpose of the OBD system is to ensure proper emission control system operation for the life of the vehicle by monitoring emissions related components and systems for deterioration and malfunction. Federal regulations establish requirements for on-board diagnostic (OBD) systems on light duty vehicles and light duty trucks.

OBD turns on a special warning light on the dashboard called a Malfunction Indicator Lamp (MIL) when it finds a fault that may cause high engine or evaporative emissions.

When the MIL is on, it means that something is wrong with the emissions system that requires repair.

If the MIL is flashing, prompt service is required.

Manufacturers cover emission control systems for varying periods, and your vehicle repairs may be covered by warranty.

The following are typical examples of a MIL:

Sample image of MIL

Or a standard symbol can be used, as in this example:

Another sample image of MIL

Frequently Asked Questions

What is OBD and how does it work?

Since the early 1980's, many vehicles have used electronics and on-board computers to control many of the engine control systems, such as fuel and ignition. Vehicle manufacturers had to develop ways to diagnose problems generated by the new electronic hardware found under the hood. Thus, auto manufacturers developed the first on-board diagnostic (OBD) systems as electronic systems replaced mechanical systems. A more sophisticated on-board diagnostic system was eventually developed, and the second-generation on-board diagnostic 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 sense the operation of specific components (e.g., the oxygen sensor) and actuate others (e.g., the fuel injectors) to maintain optimal engine control. An on-board computer, known sometimes 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 through a loss in vehicle performance or drivability. The sensors and actuators, along with the diagnostic software in the on-board computer, make up what is called “the OBD system.”

What is the connection between OBD and vehicle emissions?

OBD is an emissions control system. Older emission 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.

There are circumstances under which 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 emission-related failures and alerting the driver to the need for potential repair, vehicles can be properly repaired before emissions become a problem.

How does OBD inform drivers of problems?

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 (MIL) is illuminated. This lamp 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 car is delivered to the repair shop, a service technician can quickly retrieve the stored diagnostic trouble codes from the computer memory of the vehicle. The stored diagnostic trouble code(s) 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 of the need to 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.

Why does the Malfunction Indicator Lamp (MIL) blink or flash?

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 emission 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.

How can the Malfunction Indicator Lamp (MIL) be turned off?

Service technicians can manually turn off the MIL, but 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 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.

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?”), 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?” and “How can readiness Monitors be set?”), the vehicle will be rejected for “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.

What are readiness monitors?

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 emission components have been tested.

These monitors are included in the OBD system so overall vehicle condition can be easily assessed electronically at an emissions test station, using a data link connected to the vehicle. To ensure that the data provided to the test station reflects actual vehicle condition, the vehicle computer reports whether or not the data in the computer is current and valid. This check is referred to as “readiness”, and the vehicle will be denied an emissions test if it fails the readiness check.

A vehicle can have up to twelve 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

Depending on the vehicle and the scan tool being used, the language referring to readiness conditions can vary. Some of the possible nomenclature includes:

If monitor is “ready”If monitor is “not ready”
Not Completed

How do readiness monitors affect the automotive repair industry?

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 test station for a re-test because vehicles can not be tested if a sufficient number of monitors have not run.

Remember, vehicles are rejected when more than two non-continuous readiness monitors are “not ready” for 1996 to 2000 model year vehicles, or when more than one 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 re-test 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 re-test 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 re-test. Shops can protect their Repair Effectiveness Index (REI) by making sure that a sufficient number of monitors are “ready” and bringing vehicles back for a re-test after repairs are complete. After the vehicle passes the re-test, the vehicle can be returned to the customer.

How can readiness monitors be set?

There are many reasons the 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 car 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 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 indicate 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, 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 Component

In order to set readiness for non-continuous monitors, the vehicle must be prepared and driven in a specific manner. Monitors can not 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 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 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 .

A special note about the catalyst monitor?

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 re-test. Such vehicles presented for a re-test with the catalyst monitor “not ready” will be rejected.

How does OBD help the environment?

The intent of OBD systems is to ensure proper emission system operation for each and every vehicle and light truck during its lifetime by monitoring emission-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 emission increase occurs. OBD systems also provide automobile manufacturers with valuable feedback from their customers' vehicles that can be used to improve vehicle and emission control system designs.

How does OBD help consumers?

OBD systems are designed to alert drivers when something in the emission control system begins to deteriorate or fail.

Early diagnosis followed by timely repair can often prevent more costly repairs on both emission 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.

Are OBD-related repairs covered by warranty?

Federal law requires that the emission 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.

Can anyone service an OBD-related problem?

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 Card is an excellent resource to help select a qualified repair shop.