On-Board Diagnostics II System
OverviewThe California Air Resources Board (California ARB) began regulation of On-Board Diagnostics (OBD) for vehicles sold in California beginning with the 1988 model year. The first phase, OBD I, required monitoring of the fuel metering system, Exhaust Gas Recirculation (EGR) system, and additional emission related components. The malfunction indicator lamp (MIL) was required to light and alert the driver of the fault and the need for repair of the emission control system. The MIL must be labeled "CHECK ENGINE" or "SERVICE ENGINE SOON." Associated with the MIL was a fault code or Diagnostic Trouble Code (DTC) identifying the specific area of the fault.
The OBD system was proposed by the California ARB to improve air quality by identifying vehicles exceeding emission standards. Passage of the federal Clean Air Act Amendments in 1990 has also prompted the Environmental Protection Agency (EPA) to develop on-board diagnostic requirements. California ARB OBD II regulations will be followed until 1999 when the federal regulations will be used.
The OBD II system meets government regulations by monitoring the emission control system. When a system or component exceeds emission thresholds or a component operates outside of tolerance, a DTC will be stored and the MIL will be illuminated.
Fault detection strategy and MIL operation are associated with trips and drive cycles. Each monitor has requirements for setting and clearing DTCs and for controlling the MIL. These processes, DTC and MIL operation, descriptions of the monitors and the definition of trip and drive cycles are discussed.
The diagnostic executive is the computer program in the Powertrain Control Module (PCM) that coordinates the OBD II self-monitoring system. This program controls all the monitors and interactions, DTC and MIL operation, freeze frame data and diagnostic tool interface.
Freeze frame data describes stored engine conditions, such as state of the engine, state of fuel control, spark, RPM, load, and warm-up status at the point the first fault is detected. Previously stored conditions will be replaced only if a fuel or misfire fault is detected. This data is accessible with the diagnostic tool to assist in repairing the vehicle.
Powertrain Control Module
The center of the OBD II system is a microprocessor called the Powertrain Control Module (PCM). The PCM has a single 104 pin connector. The PCM receives input from sensors and other electronic components (switches, relays, and others). Based on information received and programmed into its memory (keep alive random access memory, and others), the PCM generates output signals to control various relays, solenoids and actuators.
Keep Alive Random Access Memory (RAM) - The Powertrain Control Module (PCM) stores information in keep alive Random Access Memory (RAM), a memory integrated circuit, about vehicle operating conditions, and then uses this information to compensate for component variability. Keep alive RAM remains powered when the vehicle ignition key is OFF so that this information is not lost.
Component Control Fail Safe Condition:
Fail Safe - This system of special circuitry provides minimal engine operation should the Powertrain Control Module (PCM), mainly the Central Processing Unit or EEPROM, stop functioning correctly. All modes of Self-Test are not functional at this time. Electronic hardware is in control of the system while in fail safe operation.
Adaptive Fuel Control Strategy
The adaptive fuel control strategy is designed to compensate for variability in the fuel system components. If, during normal vehicle operation, the fuel system is detected to be biased rich or lean, the adaptive fuel control will make a corresponding shift in the fuel delivery calculation.
Whenever an injector or fuel pressure regulator is replaced, keep alive Random Access Memory (RAM) should be cleared. This is necessary so the fuel strategy does not use the previously learned adaptive values.
To clear keep alive RAM, refer to Powertrain Control Module (PCM) Reset, Diagnostic Methods.
Failure Mode Effects Management
Failure mode effects management (FMEM) is an alternate system strategy in the Powertrain Control Module (PCM) designed to maintain vehicle operation if one or more sensor inputs fail.
When a sensor input is perceived to be out-of-limits by the PCM, an alternative strategy is initiated. The PCM substitutes a fixed value and continues to monitor the incorrect sensor input. If the suspect sensor operates within limits, the PCM returns to the normal engine running strategy.
Engine RPM/Vehicle Speed Limiter
The Powertrain Control Module (PCM) will disable all of the fuel injectors whenever an engine RPM or vehicle overspeed condition is detected. The purpose of the engine RPM or vehicle speed limiter is to prevent damage to the powertrain. In this strategy, the vehicle will exhibit a rough running engine condition. Once the driver reduces the excessive speed, the vehicle will return to the normal operating strategy.
Common OBD II Terms
Trip: A trip is defined as a Key-ON, Key-OFF event in which the Powertrain Control Module (PCM) detects the following:
1. Engine coolant temperature should exceed 70°C degrees (158°F).
2. Engine coolant temperature should change more than 20°C degrees (68°F) after starting the engine.
3. Engine speed should go over 400 rpm.
TWO TRIP DETECTION LOGIC
When the powertrain control module (PCM) detects a fault during the 1st trip, the DTC and corresponding freeze frame data are stored in the PCM's memory. The malfunction indicator lamp (MIL) will not be illuminated until the fault is detected again during the 2nd trip. Certain DTCs are capable of turning the MIL light on or blinking it during the first trip.
Diagnostic Trouble Code
Diagnostic Trouble Codes (DTCs) used in OBD II vehicles will begin with a letter and are followed by four numbers. The letter at the beginning of the DTC identifies the function of the monitored device that has failed. A P indicates a powertrain device, C indicates a chassis device, B is for body device and U indicates a network or data link code.
The first number indicates if the code is generic (common to all manufacturers), or if it is manufacturer specific. A 0 indicates generic, 1 indicates manufacturer-specific.
The second number indicates the system that is affected with a number between 1-7. The following is a list showing what numbers are assigned to each system.
1. Fuel and air metering
2. Fuel and air metering (injector circuit malfunctions only)
3. Ignition system or misfire
4. Auxiliary emission controls
5. Vehicle speed controls and idle control system
6. Computer output circuits
7. Transmission
The last two numbers of the DTC indicate the component or system where the fault is located.
Malfunction Indicator Light
When the PCM detects an emission related DTC during the 1st trip, the DTC and engine data are stored in the freeze frame memory. The MIL light is illuminated only when the PCM detects the same emission related DTC after it occurs in two consecutive trips. Once the MIL is illuminated, it will only turn off after the PCM detects three trips without any faults occurring. DTCs that would cause vehicle emissions to exceed the federal limit are capable of illuminating or blinking the MIL during the 1st trip.
Diagnostic Trouble Codes Capable of Illuminating the MIL When Detected on the 1st Trip
Misfire diagnostic trouble codes
Catalyst diagnostic trouble codes
Closed loop control diagnostic trouble codes
Fuel Trim
For OBD II vehicles, long term and short term fuel trim values will be shown in percentages. Freeze frame will also show fuel trim values as percentages. Fuel trim represents how much compensation the Powertrain Control Module (PCM) must make from ideal conditions. A higher positive value for fuel trim indicates the PCM is commanding more fuel into the engine, this can be caused by vacuum leaks, restricted fuel injectors, and others. A highly negative value indicates a lean engine command, possibly caused by leaky injectors, and others.
FREEZE FRAME DATA
When a freeze frame event is triggered by an emissions related Diagnostic Trouble Code (DTC), the Powertrain Control Module (PCM) stores various vehicle information as it existed the moment the fault occurred. The DTC number along with the engine data can be useful in aiding a technician in locating the cause of the fault. Once the data from the 1st trip DTC occurrence is stored in the freeze frame memory, it will remain there even when the fault occurs again (2nd trip) and the MIL is illuminated. Freeze frame data will not be displayed after 40 drive cycles have occurred without a fault. Data can be stored in freeze frame for only one event, however, the PCM will prioritize what data it will store. For example, an ECT fault (priority 2) was detected during the 1st trip and the freeze frame data stored. After that, a misfire DTC occurs (priority 1) in another trip; the misfire data will replace the ECT data stored in the freeze frame memory, except after a misfire or fuel injection system DTC, which will not be cleared until 80 consecutive drive cycles have occurred without a fault.
OBD II System Readiness Tests
The OBD II System Readiness Tests (SRT) are:
^ Heated oxygen sensor (HO2S) monitor
^ Catalyst efficiency monitor
^ Misfire detection monitor
^ Fuel system monitor
^ Comprehensive component monitor
^ Evaporative emission system monitor
Heated Oxygen Sensor Monitor
OBD II regulations require monitoring of the upstream heated oxygen sensor (HO2S) to detect if the deterioration of the sensor has exceeded emission thresholds. An additional HO2S is located downstream of the Warm Up- Three Way Catalytic Converter (WU-TWC), or after the pre-catalytic converter to determine the efficiency of the catalyst. Although the downstream HO2S is similar to the type used for fuel control, it functions differently. The downstream HO2S is monitored to determine if a voltage is generated. That voltage is compared to a calibrated acceptable range.
Catalyst Efficiency Monitor
The catalyst efficiency monitor is a self-test strategy within the Powertrain Control Module (PCM) that uses the downstream heated oxygen sensor (HO2S) to determine when a catalyst has fallen below the minimum level of effectiveness in its ability to control exhaust emissions.
Misfire Detection Monitor
Misfire is defined as the lack of proper combustion in the cylinder due to the absence of spark, poor fuel metering, or poor compression. Any combustion that does not occur within the cylinder(s) at the proper time is also a misfire. The misfire detection monitor detects fuel, ignition or mechanically induced misfires. The intent is to protect the catalyst from permanent damage and to alert the customer of an emission failure or an inspection maintenance failure by illuminating the Malfunction Indicator Lamp (MIL). When a misfire is detected, special software called "freeze frame" data is enabled. The freeze frame data captures the operational state of the vehicle when a fault is detected from misfire detection monitor strategy.
Fuel System Monitor
The fuel system monitor is a self-test strategy within the Powertrain Control Module (PCM) that monitors the adaptive fuel table. The fuel control system uses the adaptive fuel table to compensate for normal variability of the fuel system components caused by wear or aging. During normal vehicle operation, if the fuel system appears "biased" lean or rich, the adaptive fuel table will shift the fuel delivery calculations to remove the bias.
Comprehensive Component Monitor
The comprehensive component monitor is a self-test strategy within the Powertrain Control Module (PCM) that detects faults of any electronic powertrain component or system that provides input to the PCM and is not exclusively an input to any other OBD II monitor.
Evaporative Emissions System Monitor
The Evaporative Emission (EVAP) system monitor is a self-test strategy within the powertrain control module (PCM) that tests the integrity of the EVAP system. When a fault occurs, the EVAP system monitor is reset to "NO" and a Diagnostic Trouble Code (DTC) is set in the PCM memory. After the DTC is repaired the vehicle drive cycle must be driven to reset the monitor in preparation for Inspection Maintenance (I/M) testing.