On-Board Diagnostics II
ON-BOARD DIAGNOSTICS IIThe development and adoption of legislation calling for more stringent automotive emission requirements, initiated by the California Air Resources Board (CARB), is now part of the Federal Clean Air Act. This legislation is an extension and enhancement of previous requirements (OBD) and is known as On-Board Diagnostics II (OBD II). Federal law requires that by the 1996 model year, vehicles sold in the United States meet common standards for emission control and diagnostic capability. GEMS allows Land Rover products to meet these operating standards.
Monitoring Emissions Performance
The original OBD required that vehicles monitor operation of key components such as oxygen sensors, fuel delivery system, and the module controlling the system's powertrain. Failure of components in these systems is indicated by Malfunction Indicator Lamp (MIL) illumination and generation of a Diagnostic Trouble Code (DTC).
OBD II takes this monitoring a step further by not only checking the operation of emission components, but their performance. While the difference between monitoring operation and performance may sound small, the changes to ECM operating strategies required to accomplish this are enormous.
OBD II regulations require the vehicle's MIL to be illuminated and a DTC generated when system operating conditions are such that vehicle emissions will exceed 150% of the original emissions specification. DTCs are retrieved using the TestBook or any diagnostic scan tool. All vehicles meeting OBD II standards use a standardized 16-pin connector for engine management system diagnostics. The Diagnostic Link Connector (DLC) is located in the front passenger's footwell, near the center console.
Diagnostic Trouble Codes
OBD II requires that Diagnostic Trouble Codes for common components are provided by all manufacturers. These codes must follow the format developed in the Society of Automotive Engineer's (SAE) standard J2O12. This five-digit code consists of four numbers preceded by a single letter.
The initial letter designates the vehicle system to which the code refers. All powertrain codes begin with the letter P. The first number indicates who was responsible for the DTC definition. The number "0" indicates an SAE defined code required under OBD II while "1" indicates that this code definition is manufacturer-specific (in this case, Land Rover). The "1" codes allow manufacturers to develop diagnostic capabilities over and above those required by OBD II.
The third digit (second number) of the powertrain DTC ranges from 0 through 8 and indicates the specific system subgroup. The fourth and fifth places indicate the specific concern the DTC addresses.
The number of diagnostic codes that can be produced by the ECM has increased substantially with the introduction of OBD II. A complete list of P-codes can be found under the Trouble Code Charts section. The good news is that these codes are far more specific than those previously available. This helps technician's pinpoint the cause of a customer concern more quickly than in the past.
DTCs can be retrieved using a compatible hand held scan tool or the Land Rover TestBook Scan Tool.
Diagnostic System Manager
OBD II requires that more components be monitored for a wider range of "failures" that previously may have gone unnoticed. Because of this, you can expect the MIL to illuminate more often than in the past.
The ECM does, however, contain a special diagnostic strategy or Diagnostic System Manager (DSM) to help prevent unnecessary MIL illumination. The DSM delays vehicle self-tests, known as OBD II Monitors, until the appropriate operating conditions for the test (engine temperature, rpm, engine load conditions) are present. This provides the best indicator of fuel system and emissions control operation under real driving conditions.
The designers of OBD II also recognize that unique operating conditions can produce emissions that, for a brief period, exceed allowable levels even though engine systems are operating properly. To prevent these infrequent glitches from triggering the MIL, in most cases the DSM requires that the system exceed allowable levels on two consecutive test sequences (known as trips) before the MIL is illuminated.
The DSM software also runs the tests in a specific order. This minimizes the production of misleading DTCs. If a component or system should fail, there is no sense in performing additional tests on systems or components which rely on the failed component. They'll fail too! The GEMS diagnostic strategy doesn't bother to run tests dependent on failed components/systems until they are operating properly.
OBD II Monitoring
The OBD II system test strategy performs self-diagnostics on related systems (known as Monitors) as required by federal regulations. These OBD II Monitors are listed below. They will be covered in greater detail under the OBD II Monitors section.
- Comprehensive Component Monitor
- Fuel System Monitor
- Catalyst Efficiency Monitor
- Misfire Monitor
Warm-Up Cycle
A term used in discussing OBD II diagnostic strategy is warm-up cycle. The ECM uses the number of warm-up cycles as a counting device. After a specified number of, warm-up cycles (typically 40) DTCs that are no longer relevant to the engine operating condition are automatically erased from the ECM's memory.
This is important from a technician's standpoint because DTCs and information from a concern that illuminated a customer's MIL at one time may no longer be in the Body Control Module's (BCM) memory. If the source of the concern is no longer present (bad gasoline) and the customer has waited a long time before coming in - you won't find information to work with. On the other hand, old and irrelevant information isn't likely to be present to mislead you when searching for current concerns.
The definition of a warm-up cycle is very specific. It includes engine operation, after an engine OFF period, where engine coolant temperature rises at least 22°C (40°F) and reaches at least 71°C (160°F). It then must cool down below 71°C (160°F).
OBD II Trip
Another important concept is that of the OBD II Trip. This is defined as engine operation after an engine OFF period, where OBD II components are tested and the following monitors are completed:
- Misfire
- Comprehensive Component
- Fuel System
The completion of an OBD II Trip is required for most of the new diagnostic strategies that can produce MIL illumination.