Description and Operation

OBD-II Overview

1. Overview The California Air Resources Board (CARB) began regulating On-Board Diagnostics (OBD) for vehicles sold in California starting with the 1988 model year. The first stage, 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 illuminate and warn the driver of a malfunction and the need to repair the emission control system. Associated with the MIL was a trouble code or Diagnostic Trouble Code (DTC) identifying the specific area of the fault. The OBD system was proposed by CARB to improve air quality by identifying vehicles exceeding emission standards. The passage of the 1990 Clean Air Act Amendments also prompted the Environmental Protection Agency (EPA) to develop on-board diagnostic requirements. CARB OBD-II rules were followed until 1999, when federal standards were used. The OBD-II system complies with government standards by monitoring the emission control system. When a system or component exceeds the emission threshold or the component operates outside its tolerance, a DTC will be stored and the MIL will illuminate. The diagnostic executive is a computer program in the engine Control Module (ECM) or Powertrain Control Module (PCM) that coordinates the OBD-II self-check system. This program controls all monitors and interactions, the operation of DTCs and the MIL, freeze frame data and the scan tool interface. Freeze frame data describes the stored engine conditions, such as engine status, fuel control status, spark, speed, load and warm-up status at the moment the first malfunction is detected. Previously stored conditions will be replaced only when a fuel or misfire malfunction is detected. This data is available using a scan tool to assist in vehicle repair. The core of the OBD-II system is a microprocessor called the Engine Control Module (ECM) or Powertrain Control Module (PCM). The ECM or PCM receives data from sensors and other electronic components (switches, relays, etc.). Based on the information received and programmed into its memory (random access memory, etc.), the ECM or PCM generates output signals to control various relays, solenoids and actuators. 2. Hardware Configuration and Related Terms 1) GST (Generic Scan Tool)

2) MIL (Malfunction Indicator Lamp) - MIL transistor operation

Malfunction Indicator Lamp (MIL) is connected to the ECM or PCM terminal. Malfunction indicator lamp and battery power supply (open collector driver). On most vehicles the MIL is installed on the instrument panel. The lamp driver cannot be damaged by a short circuit. Lamps with a power rating much greater than the total MIL and lamp power rating in the tester may cause a false indication. ? With the ignition ON and engine speed (rpm) < MIN. RPM, the MIL is turned on by the driver for a bulb check. 3) MIL illumination: When the ECM or PCM detects an emission-related malfunction during the first driving cycle, the DTC and engine data are stored in freeze frame memory. The MIL illuminates only when the ECM or PCM detects the same malfunction associated with the same DTC in two consecutive driving cycles. 4) MIL extinguishing: ? Misfire and fuel system malfunctions: For misfire or fuel system malfunctions, the MIL may be extinguished if the same malfunction is not detected during three subsequent consecutive driving cycles in which conditions are similar to those under which the malfunction was first detected. ? All other malfunctions: For all other malfunctions, the MIL may be extinguished after three subsequent consecutive driving cycles during which the monitoring system is active and the malfunction is not detected, and if no other malfunction has been detected that would independently illuminate the MIL in accordance with the requirements above. 5) DTC erasure: The diagnostic system may erase a DTC if the same malfunction is not recorded again within at least 40 engine warm-up cycles and the MIL is not illuminated for that DTC. 6) Controller Area Network (CAN) communication line

• Bus Topology: Line (bus) structure
• Wiring: Twisted pair
• DLC External Cable Length: 5 m max.
• Data Transfer Rate
• Diagnostics: 500 kbps
• Service mode (reprogramming, VIN write): 500 kbps or 1 Mbps

7) Driving cycle: A driving cycle consists of engine start and engine shutdown. 8) Warm-up cycle: A warm-up cycle means sufficient vehicle operation such that the engine coolant temperature has risen at least 40 °F from engine start-up and reaches a minimum temperature of 160 °F. 9) Trip: A trip is vehicle operation (after engine shutdown) of sufficient duration and driving mode so that all components and systems are monitored at least once by the diagnostic system, except catalyst efficiency or evaporative system monitoring, which is checked at constant speed subject to the manufacturer's restrictions. All monitoring disable conditions must occur at least once during the first part of the engine start applicable FTP cycle. 10) DTC format

• Diagnostic Trouble Code (SAE J2012)
• DTCs used in OBD-II vehicles begin with a letter followed by four digits.

The first letter of the DTC identifies the system being monitored: 'P' indicates powertrain, 'C' indicates chassis, 'B' indicates body, and 'U' indicates network or data communication link. The first number indicates whether the code is generic (common to all manufacturers) or manufacturer-specific. '0' and '2' denote generic codes, '1' denotes manufacturer-specific codes. The second digit indicates the system affected, a number from 1 to 7. The list below shows which numbers are assigned to each system.

1. 1
   Fuel and Air Metering
2. 2
   Fuel and Air Metering (injector circuit malfunction only)
3. 3
   Ignition System or Misfire
4. 4
   Supplemental Emission Control Monitors
5. 5
   Vehicle Speed Control and Idle Control System Monitors
6. 6
   Computer Output Circuits
7. 7
   Infection Transmission

The last two digits of the DTC indicate the component or section of the system in which the malfunction is located. 11) Freeze frame data: When a freeze frame event is triggered by an emission-related DTC, the ECM or PCM stores various vehicle information as it existed at the moment the malfunction occurred. The DTC number together with engine data can help the technician locate the cause of the malfunction. Once the DTC data from the first driving cycle is stored in freeze frame memory, it remains there even if the malfunction occurs again (second driving cycle) and the MIL illuminates.

• Freeze Frame List

1. 1
   Calculated Load Value
2. 2
   Engine Speed
3. 3
   Fuel Trim
4. 4
   Fuel Pressure (if available)
5. 5
   Vehicle Speed (if available)
6. 6
   Engine Coolant Temperature
7. 7
   Intake Manifold Pressure (if available)
8. 8
   Closed Loop or Open Loop Operation
9. 9
   Malfunction Code

3. OBD-II Readiness Monitors 1) Catalyst Monitor. The catalyst monitor is a self-diagnostic strategy within the ECM or PCM that uses the heated oxygen sensor (HO2S) downstream to determine when the catalytic converter has dropped below the minimum efficiency level required to control exhaust emissions. 2) Misfire Monitor. A misfire is defined as the lack of proper combustion in a cylinder due to lack of spark, poor fuel metering, or poor compression. Any combustion occurring outside the cylinder at the correct time is also a misfire. The misfire monitor detects fuel, ignition, or mechanical misfires. The purpose is to protect the catalyst from irreversible damage and to alert the customer of an emissions failure or service-required condition by illuminating the MIL. When a misfire is detected, special software called freeze frame data is activated. Freeze frame data captures the operating conditions of the vehicle when a misfire-monitor fault is detected. 3) Fuel System Monitor. The fuel system monitor is a self-diagnostic strategy within the ECM or PCM that monitors the adaptive fuel table. The fuel control system uses the adaptive fuel table to compensate for normal variability of fuel system components caused by wear or aging. During normal vehicle operation, if the fuel system runs lean or rich, the adaptive value table will shift the fuel delivery calculations to remove the bias. 4) Engine Coolant System Monitor. The coolant system monitor is a self-diagnostic strategy within the ECM or PCM that monitors the ECT (engine coolant temperature) sensor and thermostat for circuit continuity, range/performance, and rationality faults. 5) O2 Sensor Monitor. OBD-II regulations require monitoring of the heated oxygen sensor (HO2S) to detect sensor deterioration that exceeds threshold values. The downstream HO2S is located after the warm-up three-way catalytic converter (WU-TWC) to determine catalyst efficiency. Although the downstream HO2S is of the same type used for fuel control, it operates differently. The downstream HO2S is monitored to determine whether voltage is generated. This voltage is compared with the calibrated permissible range. 6) EVAP System Monitor. The EVAP monitor is a self-diagnostic strategy within the ECM or PCM that checks the integrity of the EVAP system. The complete evaporative system detects leaks or combined leaks equal to or greater than a leak caused by a 0.040 inch and a 0.020 inch orifice. 7) Air Conditioning System Monitor. The A/C system monitor is a self-diagnostic strategy within the ECM or PCM that monitors the A/C system components for malfunctions. 8) Comprehensive Component Monitor. The comprehensive component monitor is a self-diagnostic strategy within the ECM or PCM that detects malfunctions of any electronic component or drivetrain system that provides input to the ECM or PCM and is not exclusively the input for any other OBD-II monitor.