Related Pages: Glossary | Diagnostic Trouble Codes (DTC)
Contents: Introduction | Code Reader/Scanner | Diagnostics | Books

Onboard Diagnostics II (OBD II or OBD2, OBD-II) is a second-generation emissions diagnostic system required by U.S. EPA on all 1996 and newer vehicles. See origins below. It includes many functions. The two main one are:
1. Activating the Check Engine light on your dash (also called a "MIL" or Malfunction Indicator Light) and storing a Diagnostic Trouble Codes - (DTCs) indicating what the problem is.
2. Storing engine parameter identification data (PID) such as throttle position, barametric pressure, coolant temperature, oxygen sensor output voltage, ...

The service industry calls the Check Engine light on your dash an "MIL" or Malfunction Indicator Light. It shows three different types of signals. Occasional flashes show momentary malfunctions. It stays on if the problem is of a more serious nature, affecting the emissions output or safety of the vehicle. A constantly flashing MIL is a sign of a major problem which can cause serious damage if the engine is not stopped immediately. In all cases a "freeze frame" of all sensor readings at the time is recorded in the central computer of the vehicle.

Most of the causes for the Check Engine light are emission control problems checks specified by the California Air Resources Board (CARB) and the Environmental Protection Agency (EPA) and will not damage your car if you wait until you get home to have them fixed. However, you should check your dashboard gauges and lights for indications of low oil pressure or overheating. These conditions mean you should pull over and shut off the engine as soon as you can find a safe place to do so.

A blinking light indicates a problem that needs immediate attention. In late-model cars, a blinking light usually indicates an engine misfire so severe that unburned fuel is being dumped into the exhaust system, where it can quickly damage the catalytic converter. If that happens, you should reduce power and have the car or truck looked at as soon as possible.

In some extreme cases, the car's computer may reduce power for you, as it tries to limit the risk of damage. This does not necessarily mean you car is going to stop running.

In a Mar. 2008 article "What to do if the "check engine" light goes on" ConsumerReports says:

"The check engine light ("service engine soon" or "check powertrain") can come on from a variety of conditions from a loose fuel cap (gas cap) too a serious problem which should be checked ASAP. It doesn't mean you have to pull the car over to the side of the road and call a tow truck. It does mean you should get the car checked out as soon as possible," says Dave Cappert of the National Institute for Automotive Service Excellence, a Virginia-based organization that tests and certifies auto technicians.

Ignore the warning, and you could end up damaging expensive components. It also can be a sign that your car is getting poor fuel economy and emitting higher levels of pollutants.

  • Look for a serious problem that requires immediate attention. Check your dashboard gauges and lights for indications of low oil pressure or overheating. These conditions mean you should pull over and shut off the engine as soon as you can find a safe place to do so.

  • Try tightening your gas cap. This often solves the problem. Keep in mind that it may take several trips before the light resets. Some vehicles have a separate indicator that warns of a loose gas cap before the condition sets off the "check engine" light.

  • Reduce speed and load. If the "check engine" light is blinking or you notice any serious performance problems, such as a loss of power, reduce your speed and try to reduce the load on the engine. For example, it would be a good idea to stop towing a trailer. Have the car checked as soon as possible to prevent expensive damage.

  • Have the code read and the problem fixed. If you want to diagnose the malfunction yourself, you can buy a scan tool at most auto parts stores. But trouble codes (e.g. "Secondary Air Injection System Malfunction") are frequently hard to resolve, unless you have a good knowledge of automotive diagnostics, you're probably better off taking the vehicle to a professional. Some automotive parts stores will read and interpret the code for you without charge. Unless there is an easy fix, they may simply refer you to a mechanic.
    GM cars with OnStar can read the code remotely.
  • Don't go for a state emissions test. In a late-model car, an illuminated "check engine" light probably is a sure sign your car will fail the test. In some states, it's an automatic failure, even if the problem was nothing more than a loose gas cap. By the way, don't bother trying to fool the inspection station by disconnecting the battery or using any other method to erase the trouble code and turn off the "check engine" light. Your vehicle's computer will let the inspection station know (For example their computer will give them a "Not Ready" indication for a component when a code has been reset and there haven't been enough engine cycles to test it again) that its codes have been erased, and you'll just have to go back again.

Some cars have a Generator light on the dash which will set trouble codes also.
Subaru's will flash trouble codes on the LED light of the oxygen moniter of the ECU (Electronic Control Unit), with the key ON (engine off). Long pulses (1.2 seconds) indicate tens and the short pulses (.2 seconds) indicate ones. See
article at

Today's automotive computers often try to compensate when there's a problem; so you may not notice deterioration in performance, In some extreme cases, the car's computer may reduce power for you, as it tries to limit the risk of damage.

(Note: The check engine light in newer cars does not come on as a service interval reminder, but in my old Mazda it came on at 30,000 miles because that was when the oxygen sensor needed to be replaced.)

Volkswagen was one of the first carmakers to install an on-board diagnostic (OBD) system starting in June 1971. They currently have more checks (over 1,000 trouble codes) than most other manufacturers. Many of these checks have been added to 2003 and later models.

The Onboard Diagnostics (OBD) defines a communications protocol and a standard connector to acquire data (Diagnostic Trouble Codes - DTCs) from passenger cars to help monitor/inspect vehicle emissions. OBD was established by the California Air Resources Board (CARB) and the Environmental Protection Agency (EPA). Detection of a problem by the Powertrain Control Module (PCM), a computer, turns on the Malfunction Indicator Lamp (MIL) (Check Engine Light) and sets a code in the computer memory which can be read by code scanners connected to a plug usually at the bottom of the dash board.

The origins of OBDII actually date back to 1982 in California, when the California Air Resources Board (CARB) began developing regulations that would require all vehicles sold in that state starting in 1988 to have an onboard diagnostic system to detect emission failures.

OBD-I was implemented on 1982-1995 GM 12 pin ALDL vehicles, 1983-1995 Ford EEC-IV vehicles, 1983-1995 Chrysler SCI vehicles, and 1991-1995 Jeeps.

Some 1994-95 vehicles are OBD-II compliant. See table below.

Onboard Diagnostics II (OBD II or OBD2, OBD-II) is a second-generation emissions diagnostic system required by U.S. EPA on all 1996 and newer vehicles (though some 1994 and 1995 model year vehicles were equipped with early versions of the system) OBDII will light a lamp called a MIL (malfunction indicator lamp), also known as the "check engine" light on the dash. The Malfunction Indicator Lamp (MIL) will illuminate anytime a vehicle's emissions exceed 1.5 times the federal test procedure (FTP) standards for that model year of vehicle. Some codes (Type B) require the condition to occur twice before the MIL comes on. Type A diagnostic trouble codes are the most serious and will trigger the MIL lamp with only one occurrence. When a Type A code is set, the OBDII system also stores a history code,

(The light comes on briefly when the ignition is frist turned on as a bulb check)
If the light is flashing, severe catalytic converter damagae and power loss will soon occur.

A scan-tool is conected to the OBDII connector to probe for OBDII data as defined by the SAE J1979 standard. The OBD-II standard allows for multiple electrical interfaces, which complicates the hardware used to interface with the vehicle.

Scanner Modes

  • Mode 1 Monitor current Powertrain data
  • Mode 2 Monitor freeze-frame data
  • Mode 3 Request emission trouble codes
  • Mode 4 Clear emission trouble codes
  • Mode 5 Request O2 sensor test
  • Modes 6 & 7 Monitor on-board test results
  • Mode 8 Bi-directional tests
  • Mode 9 Vehicle Identification

Communication protocols

  • SAE J1850 PWM - Pulse Width Modulated - (41.7k Baud) (Ford)
  • SAE J1850 VPW - Variable Pulse Width - (10.4k Baud) (General Motors)
  • ISO 9141-2 (10.4k Baud) (Chrysler, Asian, and European)
  • ISO 1430-4 (Keyword 2000/KWP 2000)
  • CAN - Controller Area Network protocol. (ISO 11898, SAE J1939) (The standard for 2008 and later models)
Diagnostic Trouble Codes
Generic (Universal, SAE) DTCs typically begin with 'P0' or 'P2' ('P' referring to Powertrain), followed by a 3-digit number (i.e., P0420 or P2420)., manufacturer specific codes format is "P1xxx".
Some manufactures also have their own set of 2 digit codes in addition to the SAE codes.
I saw one posting on the web which stated: "Some codes that are kept secret so that certain diagnosis can not be made without the equipment which is reserved for authorized dealers only."

See the DTC page

Emergency Brake may have to be set to read.

The Powertrain Control Module (PCM), monitors Emissions, engine and automatic transmission and produces Diagnostic Trouble Codes (DTC) according to the OBD standard. The check engine light comes on if a problem is detected.

Chrysler - Some 95 and later OBDII vehicles, you may be able to retrieve OBDII codes with this procedure, if equipped with digital dash. (Works in '97 Jeep wrangler but not '98 and newer.)

  • Turn the ignition key ON,OFF,ON,OFF,and finally ON again within 5 seconds. (ENGINE NOT RUNNING)
  • The codes will start to flash on the POWER LOSS or CHECK ENGINE light and may display in a digital odometer.

The "Single board engine controller (SBEC)" was an earlier version of a PCM.

Computer Code Readers & Scanners:
OBD II Code Reader/Scanners are available from $70 - $160.
They can read and erase diagnostic trouble codes; read pending codes, I/M monitors, MIL status; read and display OBD-II generic codes and their definitions.
Diagnostic Code Readers under products.
More expensive units allow you to program your PCM to increase performance. Includes improved power or mileage, optimize for 87 or 93 octane fuel, Engine Rev Limiter, Cooling Fan On/Off Temperatures, Permits use of low-temperature thermostat for cooler, denser intake charge and lower engine operating temperature, Transmission Shift Points, Transmission Shift Firmness, Top-Speed Limiter, Speedometer/Odometer Correction (not for jeep).

Diagnostic Link Connector (DLC)
Allows the Code Scanner or Scan Tool to communicate or "plug into" your vehicle's computer(s). Usually under the dash on the middle or drivers side.

Note: There are SEE: .

DTCs can be misleading. See case study on code P0401, "Low EGR Flow".
A novice mechanic may just replace the EGR valve, but in the case presented this would not actually solve the problem.

Throttle Actuator Malfunction (P1580), Mass Airflow Circuit (P0103): These codes may be set due to the driver attempting to start a VW with a low or dead battery.

Driving Cycles:
In certain cases the light will go off after a certain number of driving cycles. For example

A drive cycle or trip, by the way, is not just an ignition cycle, but a warm-up cycle. It is defined as starting the engine and driving the vehicle long enough to raise the coolant temperature at least 40 degrees F (if the startup temperature is less than 160 degrees F). See Drive Cycle below.
On some vehicles, the EVAP monitor won't run unless the vehicle has not been run for eight hours.

There are sometimes bugs in the computer causing the MIL light to come on unnecessairly. You may want to check with your dealer to see if therer are any Service Bulletin's regarding this. They can upload a new program to the computer (reflash the EEPROM).

For example, if the fuel cap (gas cap) isn't tight or the tank is filled while the key is on or the engine is idling, it can trigger a variety of codes (e.g. P0440 - Evaporative Emission Control System Malfunction, P0457 - Evaporative Emission Control System Leak Detected, P1486 - LDP has detected a pinched hose in the evaporative hose system) causing the MIL light to come on. (GM has not issued a technical service bulletin on the problem with filling the tank with the key on, but is advising its dealers and fleet customers to reflash the EEPROM with revised OBDII programming that waits to check the evaporative emissions system until the vehicle is in motion.)
It may take up to five drive cycles after the cap is tightened for the light to go out.

Pending Codes:
There are two categories of DTC's
Category A (more severe)
- Requests illumination of the MIL.
- Stores a freeze frame DTC after one failed driving cycle.

A freeze frame stores the code along with a snapshot of vehicle operating parameters to aid in identifying the problem. This block of values is referred to as Freeze Frame Data and may include engine rpm, vehicle speed, air flow, engine load, fuel pressure, fuel trim value, engine coolant temperature, intake manifold pressure and open or closed loop status.
- The MIL may be turned off and the code erased after some number (could be up to 40) of successful driving cycles.

Category B
- Sets a Pending Trouble Code when the fault is detected. - Clears a Pending Trouble Code after a certain number of successful driving cycles.
- Upgrades to category A after a certain number of failed driving cycles.

See: Deductive Diagnostics at
  Diagnostic Flow charts ... Do they always work?

An OBD Drive Cycle:
After you've "fixed" an emissions problem on your OBDII-equipped vehicle, you should perform what's called an "OBDII drive cycle." The purpose of the OBDII drive cycle is to run all of the onboard diagnostics. The drive cycle shold be performed after you've erased any trouble codes from the PCM's memory, or after the battery has been disconnected. Running through the drive cycle sets all the system status "flags" so that subsequent faults can be detected.

The OBDII drive cycle begins with a cold start (coolant temperature below 122 degrees F and the coolant and air temperature sensors within 11 degrees of one another).

NOTE: The ignition key must not be on prior to the cold start otherwise the heated oxygen sensor diagnostic may not run.

  1. As soon as the engine starts, idle the engine in drive for two and a half minutes with the A/C and rear defrost on. OBDII checks oxygen sensor heater circuits, air pump and EVAP purge.
  2. Turn the A/C and rear defrost off, and accelerate to 55 mph at half throttle. OBDII checks for ignition misfire, fuel trim and canister purge.
  3. Hold at a steady state speed of 55 mph for three minutes. OBDII monitors EGR, air pump, O2 sensors and canister purge.
  4. Decelerate (coast down) to 20 mph without braking or depressing the clutch. OBDII checks EGR and purge functions.
  5. Accelerate back to 55 to 60 mph at ¾ throttle. OBDII checks misfire, fuel trim and purge again.
  6. Hold at a steady speed of 55 to 60 mph for five minutes. OBDII monitors catalytic converter efficiency, misfire, EGR, fuel trim, oxygen sensors and purge functions.
  7. Decelerate (coast down) to a stop without braking. OBDII makes a final check of EGR and canister purge.

1994-95 OBD-II Vehicles
Ford: 1994-95 Mustang 3.8L, 1995 Ranger, 1994-95 Thunderbird 4.6L, 1995 Windstar
Lincoln: 1995 Continental 4.6L, 1995 Town Car 4.6L, 1994-95 Cougar 4.6L, 1995 Grand Marquis 4.6L

To find out if a 1994 or 1995 vehicle is OBD II compliant, check the following: The Vehicle Emissions Control Information (VECI) Label. This label is located under the hood or by the radiator of most vehicles. If the vehicle is OBD II compliant, the label will state OBD II Certified on the upper right-hand side of this label. Government Regulations require that all OBD II compliant vehicles must have a common sixteen-pin Data Link Connector (DLC). NOTE: Some 1994 and 1995 vehicles have 16-pin connectors but are not OBD II compliant, only the ones where the Vehicle Emissions Control Label states that, they are OBD II Certified.

As of 2008 ome states were considering an advanced OBD system that would allow them to do away with emissions testing. If the "check engine" light comes on, the system automatically would send a remote signal to state officials, who would contact motorists who don't have the problem corrected within a reasonable amount of time. Privacy advocates are criticizing the idea as being too intrusive.
Links: has tips for fixes
Diagnostic Code Readers under products.
Diagnostic Trouble Codes (DTC)
National OBD Clearinghouse at Web er State Univ.
parameter identification data (PID) - Glossary
  Technical Library, "OBD II Past, Present and Future",
  Sensor Guide CD ($40 at Automotive Diagnostic Software Products)
Chrysler Code Retrieval at BAT Auto Technical (BAT= Bruce, Alan and Tony)
OBD II Resources and Information at
The ABC's of OBD II at
Erase the Adaptive Memory in the TJ PCM
On Board Diagnostic Systems at
OBD2.pdf at
Automotive Buses
FAQs at actron
Diagnostic Trouble Codes (DTCs) for OBD II (OBD-2) equipped GM vehicles

OBDII Research Center at the National Center for Vehicle Emissions Control and Safety at Colorado State Univ.Z
Others at

last updated 26 Mar 2008