The impetus to control motor vehicle emissions, driven by the air quality needs of major metropolitan areas, has led to five decades of engineering innovation in the design of vehicle propulsion technologies. Because California cities (particularly Los Angeles) were hardest hit by the proliferation of vehicles and their emissions, starting in 1961 the California Air Resources Board (CARB) has been at the forefront of setting engineering standards for vehicles sold, first in California, then in the United States and now for the world automotive industry.
Modern vehicle engine operations and emissions (including evaporative losses from the fuel tank) are extensively controlled via an on-board digital microprocessor, the computer known as the engine control module or ECM (also ECU). Feedback systems, relying on sensors, allow the ECM to precisely control fuel/air mixture, spark timing and other functions. Of the many sensors on the engine and the vehicle, the primary ones are oxygen sensor(s), which monitor the combustion efficiency of the engine, and knock sensor(s), which "listen" for engine knock or pinging. The ECM, which adjusts the fuel mixture and adjusts spark timing uses the input signals from these sensors.
Computer control of the engine operations allows two benefits in addition to a precise air fuel mixture and, therefore, lower emissions. The computers can be programmed to detect faults (diagnostic trouble codes or DTCs) in their own operations, and the ECM can store this information and illuminate a malfunction indicator light (MIL), often called the Check Engine light, on the instrument cluster. The stored faults can be viewed later for diagnosis and repair.
Diagnosis of modern vehicle performance faults starts with gaining access to DTCs stored in the ECM. Start by looking for the 16-pin OBD II plug (diagnostic link connector or DLC) under the dashboard. By law, it is always located within reach of the driver.
Once the diagnostic scan tool is connected to the DLC, follow the instructions on the scan tool screen. These usually include:
- Select diagnosis program
- Select correct vehicle (scan tool should find this automatically)
- Interrogate and record DTCs (fault codes) present in ECM memory
In many cases, it is best to clear DTCs. Then drive the vehicle and retest it. Follow the diagnostic and repair instructions on the scan tool screen. DTCs for the vehicle power train begin with a P and are followed by 4 digits. (They are also referred to as P-codes). During testing, the scan tool displays each P-code and a brief description of the fault. There are different kinds of faults indicated by DTCs:
- Plausibility: If a sensor's output value is outside the expected range, the signal from the sensor is considered not plausible and a fault is set.
- Power, ground, continuity: If power or ground is missing or continuity is lacking in a particular circuit or system, a fault is set.
- Fault in sensor or module: If a sensor or module tests defective, a fault is set.
You should note that even though the scan tool displays specific DTCs, additional diagnosis is advisable. For example, if a DTC indicates a non-functional oxygen sensor heater, the simple explanation may be a blown fuse. In other words, use common sense and do not ignore the obvious when diagnosing problems.
Working at dashboard, open OBD II connector access door.
Plug scan tool into OBD II connector.
Follow directions supplied with scan tool to interrogate ECM fault memory. Read fault code, diagnose problem, then clear fault code when complete.