Introduction To OBD II
OBD II is simply the latest in a fairly long line of regulations designed to bring about cleaner air. In general, OBD II regulations require more control of vehicle emissions and more precise monitoring of emission-related devices. Technicians at independent garages must be able to access the OBD II system without the use of proprietary tools available only to dealers. The technology required to develop this new program was outside CARB's field of expertise, so they enlisted the Society of Automotive Engineers (SAE) to establish the recommended practices. The SAE involvement brought OBD II to the attention of the EPA, which soon adopted portions of the new regulations nationwide. Compliance with OBD II requirements was gradually phased into production beginning with some new engine/emissions electronic control systems for the 1994 model year. By 1996, all vehicles, depending on their gross vehicle weight rating, sold as 1996 or later models had to have an OBD II system.OBD II Requirements
Technicians benefit from OBD II, which standardizes electronic diagnosis. Technicians can use the same inexpensive, generic scan tool without special adapters to test any OBD II vehicle that comes into the shop. The OBD II regulations mandate: Onboard monitoring of emissions-related components; a universal Data Link Connector (DLC); a standard DLC location; a standard communications protocol between the vehicle and a scan tool; a standard list of Diagnostic Trouble Codes (DTCs); storing a freeze frame of data values in memory when a DTC fault occurs; scan tool access to DTC records, freeze frame data, readiness tests, and DTC clearing; a standard for alphanumeric descriptors used on scan tool data and a standard method for new terms.System Monitors
OBD II diagnostic systems perform many dedicated emissions systems monitor tests. Some of these monitors run continuously, which means they are always operating whenever the vehicle is running. These continuous monitors include: misfire detection, fuel trim, and comprehensive component monitors. There are several other monitors, which are known as non-continuous monitors, that perform once per trip. They include, catalyst and heated catalyst, oxygen sensor and oxygen sensor heater, evaporative emissions, secondary air, and EGR system monitors.Operating conditions, called enabling criteria must exist before a monitor can run its test. The enabling criteria are not the same for all of a vehicle's monitors and the criteria is specific for each monitor. The enabling criteria may include all or some of the following operating conditions: vehicle and engine speed, engine load, throttle position, engine coolant temperature at start up and during operation, and elapsed time since startup. The term, trip, describes the portion of a complete drive cycle that satisfies a particular monitors enabling criteria. The complete drive cycle contains the enabling criteria necessary for all monitors to be run.
The system reports via the scanner the non-continuous monitors test status. A test status of YES means the test has been run and a test status of NO means the test has not been run. The monitor test status does not refer to whether the system or component passed or failed the test only whether the test has been run. If the system codes are cleared or the battery is disconnected all the monitor status will be NO until the specific criteria occurs to enable the monitors to be run. In these instances, when the all monitor status is NO, the vehicle must be driven on a complete drive cycle that runs all monitor tests.
Data Link Connector
The OBD II DLC is a 16-pin connector with pins arranged in two rows of 8, numbered 1 to 8 and 9 to 16. The DLC is D-shaped, like a computer printer cable connector, so the two halves will only fit together one way. Ideally, technicians can connect a scan tool to the DLC using one hand, while seated in the driver's seat. Although out of the normal line of sight of the driver and vehicle occupants, the DLC should be clearly visible to a technician in a crouched position. The DLC cannot be hidden behind trim panels and must be accessible without the use of tools. However, the EPA granted one-time exemptions for DLC location to a number of vehicle manufacturers. Consequently, on certain models the DLC may be on the passenger side, behind the ashtray, in the center console, or in some other location. If so, the cover is usually marked OBD or OBD II for easy recognition.Any "generic" scan tool must be able to connect to the DLC and access the diagnostic data stream. Seven of the 16 pin positions of a DLC were given mandatory assignments. Use of the remaining 9 pins is at the vehicle manufacturer's discretion. Most systems use only five of the seven dedicated pins. Terminals 7 and 15 are for systems that transmit data conforming to the European standards established by ISO-1941-2 regulation. This is an alternate communications network to the one defined by SAE J1850 that communicates on the DLC at terminals 2 and 10.
A major SAE contribution was the development of a standard communications protocol, J1850. This protocol, along with the International Standards Organization ISO-1941, establishes the method that a vehicle OBD system shall use to send information to a scan tool. SAE also developed J1979, which is a recommended practice for software modes that are used by a generic scan tool. The OBD II regulations cover a broad spectrum and have a major impact on vehicle design and service. To be OBD II compliant, a system must detect increases in emission levels that cause components to deteriorate. The malfunction indicator lamp (MIL) must light when a catalytic converter allows hydrocarbon (HC) emissions to increase greater than 1.5 times the level allowed by Federal Test Procedure (FTP) standards. The other emission control systems and equipment have similar requirements.
Diagnostic Trouble Codes
All OBD II DTCs are to be universal, used by all manufacturers, so that technicians at independent shops do not have to learn multiple DTC styles. These OBD II DTCs are in five-character, alphanumeric format. The first character, a letter, defines the system where the code was set. The second character, a number, reveals whether it is an SAE or a manufacturer defined code. The remaining three characters, all numbers, describe the type of the malfunction.Standard Terminology
Unfortunately, with the tremendous growth of sophisticated electronic control systems came an equal growth in the number of terms, often many different terms for the same system or part. The SAE helped bring some order to this with the document, J1930. It standardizes the electrical/electronic systems and diagnostic terms, definitions, abbreviations, and acronyms. It also provides a method for vehicle manufacturers to use when constructing new terms for new components and systems. The J1930 document focuses on diagnostic terms applied to electrical and electronic systems. It also contains the related mechanical terms, definitions, abbreviations and acronyms. As the list of terms grows or changes, the SAE will amend J1930.OBD II System Operation
Comprehensive Component Monitor
The OBD II diagnostic system continuously monitors all engine and transmission sensors and actuators for shorts, opens, and out-of-range values, as well as values that do not logically fit with other powertrain data (rationality). On the first trip during which the comprehensive component monitor detects a failure that results in emissions exceeding a predetermined level, the PCM stores a diagnostic trouble code (DTC), illuminates the malfunction indicator lamp (MIL), and stores a freeze frame.System Monitors
The OBD II diagnostic system also actively tests some systems for proper operation while the vehicle is being driven. Fuel control and engine misfire are monitored continuously, while catalyst efficiency, EGR operation, EVAP integrity, O2S response, and the oxygen sensor heaters are tested once per trip. When any of the system monitors detects a failure that causes emissions to exceed a predetermined level on two consecutive trips, the PCM stores a freeze frame.Fuel Control
This monitor sets a DTC if the long term Fuel Trim (FT) is excessively high or low anytime after the engine is warmed up, which indicates the loss of fuel control. This is always the case when the long term FT reaches its limit (+30% or -30%).Engine Misfire
This monitor uses the CKP sensor signal to continuously detect engine misfires both severe and non-severe. If the misfire is severe enough to cause catalytic converter damage, the MIL flashes on and off as long the severe misfire is detected.Catalytic Converter
This monitor compares the signals of the two upstream H2OSs to the signal from the downstream H2OS to determine the ability of the catalyst to store free oxygen. If the oxygen storage capacity of the converter is sufficiently degraded, a DTC is set.EGR System
This monitor uses the MAP sensor signal to detect changes in intake manifold pressure as the EGR valve is commanded to open and close. If the pressure changes too little or too much, a DTC is set.EVAP System
This monitor first energizes the EVAP vent solenoid to block the fresh air supply to the EVAP canister. Next, the EVAP purge solenoid energizes to draw a slight vacuum on the entire EVAP system, including the fuel tank. Then, the EVAP purge solenoid is turned off to seal the system. The monitor uses the fuel tank (EVAP) pressure sensor signal to determine if the EVAP system has any leaks. If the vacuum decays too rapidly, a DTC is set. In order to run this monitor, the engine must be cold (below 86ºF/30ºC) and the fuel level must be between 1/4 and 3/4 full.Oxygen Sensors
This monitor checks the maximum and minimum output voltage, as well as switching and response times for all of the oxygen sensors. If an oxygen sensor signal remains too low or too high, switches too slowly, or not at all, a DTC is set.Oxygen Sensor Heaters
This monitor checks the time from cold start until the oxygen sensors begin to provide a reliable feedback signal to the PCM. If the time is too long, a DTC is set. Battery voltage is continuously supplied to the oxygen sensor heaters whenever the ignition switch is on.Monitor Readiness Status
The monitor readiness status indicates whether or not a particular OBD II diagnostic monitor has been run since the last time that DTCs were cleared from PCM memory. If the monitor has not yet run, the status displays on the scan tool as "NO." If the monitor has been run, the status displays on the scan tool as "YES." This does not mean no faults were found, only that the diagnostic monitor has been run. Whenever DTCs are cleared from memory or the battery is disconnected, all monitor readiness status indicators are reset to "NO." Monitor readiness status indicators are not needed for the comprehensive component, fuel control, and engine misfire monitors because they run continuously.Warm-Up Cycle
Warm Up cycles are used by the PCM for automatic clearing of DTCs and freeze frame data as described below. To complete one warm up cycle, the engine coolant temperature must rise at least 40ºF (22ºC) and reach a minimum of 160ºF (71ºC).Trip
A trip is a key-on cycle in which all enable criteria for a particular diagnostic monitor are met and the diagnostic monitor is run. The trip is completed when the ignition switch is turned off.Drive Cycle
Most OBD II diagnostic monitors run at some time during normal operation of the vehicle. However, to satisfy all of the different trip enable criteria and run all of the OBD II diagnostic monitors, the vehicle must be driven under a variety of very specific conditions.Freeze Frame Data
A freeze frame is a miniature "snapshot" of one frame of data that automatically stored in the PCM memory when an emissions-related DTC set. If a DTC for fuel control or engine misfire is stored at a later time, the newest data is stored and the earlier information is lost.Storing And Clearing DTCs & Freeze Frame Data, Turning The MIL On & Off
One Trip Monitors
A failure on the first trip of a "one trip" emissions diagnostic monitor causes the PCM to immediately store a DTC and freeze frame, and turn on the MIL. All comprehensive component monitor faults require only one trip.Two Trip Monitors
A failure on the first trip of a "two trip" emissions diagnostic monitor causes the PCM to store a temporary DTC. If the failure does not recur on the next trip, the temporary DTC is cleared from memory. If the failure does recur on the next trip, the PCM stores a DTC and freeze frame, and turn on the MIL. All the system monitors are two trip monitors. Engine misfire which is severe enough to damage the catalytic converter is a two trip monitor, with the additional condition that the MIL flashes while the severe misfire is occurring.Automatic Clearing
If the vehicle completes three consecutive "good trips," three consecutive trips in which the monitor that set the DTC is run and passes, the MIL is turned off, but the DTC and freeze frame remain stored in PCM memory. If the vehicle completes 40 warm-up cycles without the same fault recurring, the DTC and freeze frame are automatically cleared from the PCM memory.Manual Clearing
All stored DTCs and freeze frame data can be erased using the scan tool. The MIL, if lit, is turned off as well. Although it is not the recommended method, DTCs and freeze frame data are also cleared if the battery power supply to the PCM is disconnected.DTC, MIL & Freeze Frame Summary
Since there are so many ways to set and clear DTCs and to turn the MIL on and off, here are some things to keep in mind:- A failed test always sets a DTC.
- Type A DTC lights the MIL upon first detection and records a freeze frame of the first occurrence.
- Failed tests for a type B DTC light the MIL and record a freeze frame if a frame is empty.
- Three consecutive trips that meet the test enabling criteria without a detected failure turn off the MIL.
- The PCM clears a DTC when the required number ignition cycles, 40 cycles for an emission DTC, have occurred without a recurring failure. Nonetheless, if the fault is a type B, non-catalyst damaging misfire or fuel trim DTC, it may remain as a pending code for up to 80 cycles.
EPROM & EEPROM
Permanent computer memory is called read-only memory. This memory contains the programming and look-up tables that calibrate the computer to the specific vehicle and emissions application. The vehicle computer can read but cannot change the contents of the PROM. In the past, PROM chips had to be replaced in order to update or correct a flaw in the program or look-up table. On computers with the PROM hard wired into the circuit board, it meant replacing the entire computer. Erasable PROMs allow technicians to change programming and look-up table information without replacing the actual PROM chip, so that emissions and driveability problems can be fixed without expensive hardware replacement.Two types of PROM chips are in use today. The first, erasable programmable read only memory (EPROM) can be erased when a small window on the top of the chip is exposed to ultraviolet light. After exposure to the ultraviolet light for a specific amount of time, the EPROM can be reprogrammed. However, this requires the removal of the EPROM and reprogramming by the vehicle manufacturer. When an EPROM does not have to be removed and can be programmed through the computer data link connector (DLC) it is called EEPROM. The second, electronically erasable programmable read only memory (EEPROM) is a feature that is not OBD II specific. Some systems that are not OBD II compliant have EEPROM. The advantage of the EEPROM is it can be updated or corrected without having to remove or replace it. Updates or corrections are communicated through the computer DLC.
California Air Resources Board (CARB) regulations require that any re-programmable computer system include write-protection procedures and hardware to prevent tampering. The EPA requires that any re-programmable PCM be tamper resistant and conform to SAE J2186 EE data link security standards.
There are three methods of programming: direct, remote, and off-board. Direct programming requires a direct connection the manufacturer's data terminal. This is usually only performed at the factory or dealership. Remote programming involves transferring program data through the vehicle DLC by way of a scan tool. Off-board programming is performed with the PCM removed from the vehicle. To program a PCM off-board, some manufacturers use an adapter and scan tool. The procedure can then be performed using one of the previously mentioned direct or remote methods. The programming data is most often downloaded via telephone modem or, in some cases, CD-ROM.
The EPA requires that vehicle manufacturers make the data available to the aftermarket and generic scan tool functions include the ability to perform the update. A personal computer (PC) may be required to download the data. It is most important, when performing any update, to make good connections and to properly identify the vehicle so that correct data is transferred to the PCM. An adaptive-strategy relearn-procedure should always be performed after programming an EEPROM.
Additional OBD II System Operation
The following OBD II system operations are not used on all vehicles. The explanations of secondary air injection and diagnostic trouble code reporting are included to provide a more complete understanding of OBD II system operation.Secondary Air Injection System Diagnostics
The secondary air injection system uses a pump to force air into the exhaust stream under certain conditions. This promotes oxidation, or burning, of HC and CO until the system achieves closed loop operation.When the HO2S is active, it delivers a lean, or lean low voltage, signal while the secondary air is delivered in the exhaust. Expect upstream HO2S readings on the 0 to 200 mV range with secondary air.
The system uses an electric air pump that draws high electrical current and must be energized through a relay. Both the power and control circuits are checked by the PCM for open and short circuit faults.
The PCM uses both a passive and an active test to monitor the function of the secondary air injection system. The passive test looks for an expected low voltage signal from the upstream HO2S while secondary air is being delivered to the exhaust. The passive test also checks the HO2S for a properly switching signal after the air injection system is turned off.
If the passive system indicates a pass result, no further action is taken. If the passive test fails or is inconclusive, that is a lean HO2S signal not observed or unclear, the system proceeds to the active test. During the active test the PCM turns on the air pump during closed loop operation and observes the HO2S response. As in the passive test, a lean HO2S signal shows that the air injection system is functioning. An increased short-term fuel trim value also indicates that the system is functioning.
The secondary air injection monitor requires failure of an active test on two consecutive trips to illuminate the MIL and record a DTC. The PCM runs an active test only when a passive test fails or the results were inconclusive.
Diagnostic Trouble Codes
A DTC is directly related to a fault discovered during a diagnostic test. The PCM records a DTC based on a failed test during one or more trips. Certain failures must occur during two consecutive trips before a DTC is recorded. When a fault is detected, the related DTC is classified as one of four types: Type A, Type B, Type C, and Type D.Any of the four DTC types set when the test enable criteria is met, the test is run, and it results in a detected failure. However, Type B DTCs require two consecutive trips with a failure detected to set a DTC and turn on the MIL. Some failures that are typically type B DTCs have certain conditions when they will illuminate the MIL and set a code on the first trip.
Type A
- Emissions related.
- Illuminates the MIL on the first trip, flashes the MIL if it is catalyst damaging.
- Stores a freeze frame of the first failure if memory is empty.
- Stores in history on the first trip (some models).
- Stores a fail record.
- Updates the failure record each failure.
Type B
- Emissions related.
- Pending after one trip failed.
- Pending code disarmed after one trip passed.
- Illuminates MIL after second trip failed.
- Stores freeze frame data of the first trips failure, if freeze frame is empty.
- Stores in history on the second trip failed (some models).
- Stores a fail record on the first failure.
- Updates the fail record each failure.
Type C
- Failure does not cause emissions greater than 1.5 times FTP standard.
- Illuminates a service lamp or message (not the MIL) on the first trip.
- Does not store a freeze frame.
- Stores in history on the first trip (some models).
- Stores a fail record.
- Updates the fail record each failure.
Type D
- Failure does not cause emissions greater than 1.5 times FTP standard.
- Does not illuminate any service lamp or message.
- Does not store a freeze frame.
- Stores a fail record.
- Updates the fail record each failure.
Diagnostic Executive Or Task Manager
The diagnostic executive or task manager is the computer programming that contains the strategy for the following tasks:- Determines whether the enabling criterion necessary for a monitor to run exists. Checks for conflicting situations, conflicting situations include component faults and other monitors presently running.
- Commands the comprehensive component and emission systems monitors.
- Sets diagnostic trouble codes (DTCs) in memory according to code classification and status. Decides whether the malfunction indicator light (MIL) is off, on, or flashing.
- Prioritizes freeze frame data storage and erasure (including failure record management on some vehicles).