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Table of Contents What is OBDII(On-Board Diagnostic)?Where is the data link connector(DLC) What is Diagnostic trouble codes(DTCS)What is Malfunction Indicator Light(MIL)... what is Freeze Frame Data?

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basic OBD knowledgecontent includes:What is OBD,What are the usualreasons for an OBD Failure, What is a MalfunctionIndicator Light (MIL) and so on

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Table of ContentsWhat is OBDII(On-Board Diagnostic)?Where is the data link connector(DLC)What is Diagnostic trouble codes(DTCS)What is Malfunction Indicator Light(MIL)?what is Freeze Frame Data?what is OBDII Readiness Monitors?What does it mean when my vehicle's MIL Light remains on?What is the purpose of the Malfunction Indicator Light (MIL)?What should be done when the MIL Light remains ON?What are the usual reasons for an OBD diagnostic Failure?related obd diagnostic toolsmore obd articlesGuestbook Comments

What is OBDII(On-Board Diagnostic)?OBDII(On-Board Diagnostic) systems are in most cars and light trucks on the road today.During the '70s and early 1980s manufacturers started using electronic means to controlengine functions and diagnose engine problems. This was primarily to meet EPA emissionstandards. Through the years on-board diagnostic systems have become more sophisticated.OBD-II, a new standard introduced in the mid-'90s, provides almost complete engine controland also monitors parts of the chassis, body and accessory devices, as well as the diagnosticcontrol network of the car.To combat its smog problem in the LA basin, the State of California

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started requiring emission control systems on 1966 model cars. The federal governmentextended these controls nationwide in 1968.Congress passed the Clean Air Act in 1970 and established the Environmental ProtectionAgency (EPA). This started a series of graduated emission standards and requirements formaintenance of vehicles for extended periods of time. To meet these standards,manufacturers turned to electronically controlled fuel feed and ignition systems. Sensorsmeasured engine performance and adjusted the systems to provide minimum pollution. Thesesensors were also accessed to provide early diagnostic assistance.

At first there were few standards and each manufacturer had their own systems and signals.In 1988, the Society of Automotive Engineers (SAE) set a standard connector plug and set ofdiagnostic test signals. The EPA adapted most of their standards from the SAE on-boarddiagnostic programs and recommendations. OBD-II is an expanded set of standards andpractices developed by SAE and adopted by the EPA and CARB (California Air ResourcesBoard) for implementation by January 1, 1996.

Where is the data link connector(DLC)The DLC(Data Link Connector or Diagnostic LinkConnector) is the standardized 16-cavity connectorwhere diagnostic scan tools interface with thevehicle's on-board computer.the DLC is usuallylocated 12 inches from the center of the instrumentpanel (dash),under or around the driver's side formost vehicles.if data link connector is not locatedunder dashboard,a label should be there tellinglocation.for some asian and european vehicles,thedlc is located behind the ashtray and the ashtray must be removed to access the connector.ifthe dlc cannot be found,refer to the vehicle's service manual for the location.

What is Diagnostic trouble codes(DTCS)OBDII Diagnostic Trouble Codes(DTCS) are codes that are stored by the on-board computerdiagnostic system in response to a problem found in the vehicle.these codes identify aparticular problem area and are intended to provide you with a guide as to where a fault mightbe occurring within a vehicle. OBDII Diagnostic Trouble Codes consist of a five-digitalphanumeric code.the first character,a letter,identifies which control system sets the code.theother four characters,all numbers,provide additional infomation on where the DTC originatedand the operating conditions that caused it to set. pls see the above example to illustrate thestructure of the digits.

What is Malfunction Indicator Light(MIL)?Malfunction Indicator Light(MIL) - Malfunction Indicator Light(Service Engine Soon, CheckEngine) is a term used for the light on the instrument panel. It is to alert the driver and/or therepair technician that there is a problem with one or more of vehicle's systems and may causeemissions to exceed federal standards.if the MIL illuminates with a steady light,it indicates thata problem has been detected and the vehicle should be serviced as soon as possible undercertain conditions,the dashboard light will blink or flash.this indicateds a severe problem andflashing is intended to discourage vehicle operation.the vehicle onboard diagnostic systemcan not turn the MIL off until the necessary repairs are completed or the condition no longerexists

what is Freeze Frame Data?Freeze Frame Data - When an emissions related fault occurs,the OBDII system not only setsa code but also records a snapshot of the vehicle operating parameters to help identifying theproblem. This set of values is referred to as freeze frame data and may include importantengine parameters such as engine RPM, vehicle speed, air flow,engine load,fuelpressure,fuel trim value,engine coolant temperature,ignition timing advance,or closed loop

status.

what is OBDII Readiness Monitors?An important part of a vehicle's OBDII system is the Readiness Monitors,which are indicatorsused to find out if all of the emissions components have been evaluated by the OBDII system.They are running periodic tests on specific systems and components to ensure that they areperforming within allowable limits.Currently,there are eleven OBDII readiness monitors (or I/Mmonitors) defined by the U.S. Environmental protection agency(EPA). Not all monitors aresupported by all vehicles and the exact number of monitors in any vehicle depends on themotor vehicle manufacturer's emissions control strategy.Continuous Monitors Some of thevehicle components or systems are continuously tested by the vehicle's OBDII system, whileothers are tested only under specific vehicle operating conditions.the continuously monitoredcomponents listed below are always ready:1.Misfire2.Fuel System3.Comprehensive Components(CCM)Once the vehicle is running,the OBDII system is continuously checking the abovecomponents,monitoring key engine sensors,watching for engine misfire,and monitoring fueldemands. Non-Continuous Monitors - unlike the continuous monitors,many emissions andengine system components require the vehicle to be operated under specific conditionsbefore the monitor is ready. These monitors are termed non-continuous monitors and arelisted below:4.EGR System5.O2 Sensors6.Catalyst7.Evaporative System8.O2 Sensor Heater9.Secondary air10.Heated Catalyst11.A/C system

What does it mean when my vehicle's MIL Lightremains on?The OBD system has indicated that it detected that your vehicle has an emission problem byilluminating the MIL light and has stored a Diagnostic Trouble Code (DTC) in the vehicle'scomputer memory.Sometimes, the OBD system will automatically turn off the MIL light if the conditions thatcaused the problem are no longer present. This will happen when the OBD system evaluatesa component or system three consecutive times and no longer detects the initial problem.For example, if a gas cap is not properly tightened after refueling, the OBD system maydetect vapor leakage and turn on the MIL light. If the gas cap is tightened,the OBD system willrecognize this and the MIL light will be turned off after a few days of driving.

What is the purpose of the Malfunction IndicatorLight (MIL)?During everyday operation, the MIL light will illuminate for a few seconds when a vehicle is firststarted and extinguish when the engine is running. When the MIL light remains on duringnormal vehicle operation, the OBD system has detected a problem with the vehicle's engine,transmission or emission control system. Do not ignore this warning!!

NOTE: If the MIL light is continuously ON when the vehicle undergoes an emission test, theresult will be a FAIL.

What should be done when the MIL Light remainsON?

The OBD system is alerting you that the vehicle should be looked at soon by a CertifiedEmission Repair Technician. there is a better way you can do it .buy a auto code readerdiagnostic for yourself.Timely diagnosis may save you money in three ways:

Small problems are identified before they become major expenses

A faulty component may be covered by the vehicle's emission warranty

OBD test results allow a Certified Emission Repair Technician to accurately pinpointproblems that could reduce costly diagnostic fees

What are the usual reasons for an OBD diagnosticFailure?A vehicle will fail an OBD emissions test for the following reasons:

(1) MIL light remains ON while vehicle is running

(2) OBD Monitors are Not Ready

(3) OBD Connector is missing, damaged or inac- cessible

(4) MIL light does not operate (OBD system is not functioning or the bulb is burned out)

(5) OBD Communication failure

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