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What is OBDII and How is it Used?
Since the early 1980s, car manufacturers have used a computer-based PCM (power-train-control module) to manage the fuel and ignition system in the car. Tightened emissions standards have made it compulsory for car makers to put complicated pollution-control systems under the PCM's control as well. As the PCM system grew in complexity, engineers added onboard diagnostics to help technicians troubleshoot malfunctioning engines. Today, onboard computers play such an important role in reducing vehicle emissions that, since the 1994 model year, the US EPA (Environmental Protection Agency) has established requirements for OBD (onboard-diagnostic) systems. The latest system, OBD II, has been standard equipment on US light-duty cars and trucks since the 1996 model year. (Delong, 2002) There are nine modes of operation described in the original J1979 OBD II standard. They are:
- Mode 1: Show sensor current data
- Mode 2: Show freeze frame data (A snapshot of data taken when the last diagnostic trouble code was set)
- Mode 3: Show stored trouble codes
- Mode 4: Clear trouble codes and stored values
- Mode 5: Test results, oxygen sensors
- Mode 6: Test results, noncontinuously monitored
- Mode 7: Show pending trouble codes
- Mode 8: Special control mode
- Mode 9: Request vehicle VIN and other information
Vehicle manufacturers are not required to support all modes, and each manufacturer may define additional modes above Mode 9 for other information.
If you think of your car's engine as a device that converts gasoline chemical energy into mechanical energy, then you might believe that the engine that powered your car to work today and the engine that went into the first car Henry Ford built in 1903 are almost the same. In the eyes of the EPA, however, the engines differ significantly. For example, the hydrocarbon emissions of a subcompact car built in 1988 are less than one-thirtieth of a 1921 Ford Model T (Reference 1). This statistic is even more impressive when you consider that the subcompact has four times the horsepower and nearly twice the gas mileage, and it weighs 300 lbs more than the Model T. As the EPA's emissions regulations become more stringent, car makers continue to turn to more sophisticated software running on more powerful microprocessors to monitor and control your car's performance.
The two biggest sources of pollution from vehicles are the fuel system and the exhaust. In an ideal world, car's engine would burn gasoline with oxygen from the air to create heat, carbon dioxide, and water. Unfortunately, the combustion process also releases unburned hydrocarbons, nitrogen-oxide gases, and carbon monoxide. The good news is that today's engines and fuel mixtures are so effective that most hydrocarbon emissions are a product of evaporative losses, which come from the car's fuel tank and when car is refueled.
EPA OBD II guidelines require car manufacturers to monitor all emissions-related systems in a vehicle. These systems may include the car's air conditioner, catalytic converter, evaporative-emissions-control system, EGR (exhaust-gas-recirculation) system, fuel-delivery system, heated-oxygen sensor, and secondary-air-injection system (Reference 2). OBD II also requires the detection of engine misfires, which can damage the catalytic converter. Every time you take a trip, the OBD system tests car's emissions systems to make sure the car is not polluting beyond acceptable limits. The EPA FTP (Federal Test Procedure) sets allowable emissions levels for light-duty cars and trucks. If the OBD II system detects an emissions component that has failed or has deteriorated to the point at which the vehicle's emissions may rise to more than 1.5 times the FTP standard, the PCM must illuminate the malfunction-indicator lamp. In some cars, this indicator is called the Check Engine or Service Engine Soon light. When the malfunction-indicator lamp is on, the PCM also stores a DTC (diagnostic trouble code) in memory along with the operating conditions at the time of the failure in a "freeze frame." When a sensor malfunctions and no longer sends valid information, the PCM substitutes default values for the faulty sensor's signal so you can still drive your car and seek repairs. (Kirton, 2002)
Today's cars comes equipped with onboard-refueling-vapor-recovery and evaporative-emissions-control systems to minimize hydrocarbon emissions from the fuel tank. The onboard-refueling-vapor-recovery system prevents hydrocarbon vapors from escaping into the atmosphere when you refuel your car by trapping the vapors in an evaporative emissions canister. The canister is filled with activated carbon, which stores hydrocarbon vapors for the evaporative-emissions-control system until the PCM purges the vapors to the engine for burning. The PCM must perform several tests on the evaporative-emissions-control system. One test must check for leaks as small as 0.020 in. in diameter in the fuel system. The PCM begins this "small-leak" test by closing the valve to the vapor canister and drawing a small vacuum in the fuel tank. If the PCM fails to detect a vacuum, a large system leak is likely. One possible cause of a large leak may simply be a missing gas cap. Once the PCM verifies that the proper vacuum level exists, it waits a certain amount of time and measures the vacuum again. If the PCM records an insufficient vacuum, it stores a DTC and lights the malfunction-indicator lamp. Measuring differences in vacuum levels to detect small leaks requires the PCM to measure precisely how much air is in the fuel tank. Today, car makers use very accurate fuel gauges to determine the amount of fuel and, therefore, air in the fuel system. (Douglas, 1994, pg 33)
The catalytic converter is one of the most expensive emissions-reducing components on your car, and misfires are its worst enemy. A misfire is a result of incomplete combustion, and one cause may be a defective spark plug. Misfires send unburned hydrocarbon gases into the exhaust and into the atmosphere. But the unburned fuel also travels through the catalytic converter and may permanently damage it. For this reason, automotive engineers spend a lot of time designing ways to detect misfires. The most common approach takes advantage of the fact that each cylinder contributes to the rotational velocity of the crankshaft. If a cylinder misfires, the crankshaft velocity decreases momentarily until the next cylinder in the firing order takes over. The PCM uses a sensor on the crankshaft to compute the rotational speed and another sensor on the camshaft to determine which cylinder misfired. (Delong, 2002)
Most misfire-detection systems are low-data-rate systems. Low-data-rate systems meet FTP monitoring requirements on most of today's engines and "full-range" misfire monitoring requirements on four-cylinder engines. Currently, regulations require misfire detection at 0 to 55 mph. Full-range misfire detection extends to full throttle. OBD II regulations from the 2002 model year will require full-range misfire monitoring on most six- and eight-cylinder engines. To meet the new requirements, engineers use high-data-rate misfire monitoring systems with 18 position references per engine revolution as opposed to just one in low-data-rate systems. The greater resolution allows the PCM to use more sophisticated algorithms for detecting misfires. One algorithm "learns" the normal pattern of cylinder accelerations. From this normal pattern, the PCM can detect deviations or misfires from the positional data. Another algorithm detects a continuously misfiring cylinder by filtering out "noise" in data in the form of crankshaft torsional vibration. An OBD II equipped car limits air emissions by warning you when the car's emissions systems fail. In the future, an OBD III system may transmit the car's emissions status to a state regulatory agency without your knowledge.
OBD II diagnostic equipment utilizes these existing systems to display data that was otherwise intended to report information specifically generated for emissions testing. Most diagnostic equipment will offer a range of sensors as well as allow you to reset the CEL (Check Engine Light), MIL (Malfunction Indicator Lamp).
Readily available 'generic' scan data provides an excellent foundation for OBD II diagnostics. Recent enhancements have increased the value of this information when servicing newer vehicles. If you don’t have a good starting point, drivability diagnostics can be a frustrating experience. One of the best places to start is with a scan tool. The question asked by many is, “Which scan tool should I use?” In a perfect world with unlimited resources, the first choice would probably be the factory scan tool. Unfortunately, most technicians don’t have extra-deep pockets. That’s why my first choice is an OBD II generic scan tool. I’ve found that approximately 80% of the drivability problems I diagnose can be narrowed down or solved using nothing more than OBD II generic parameters. And all of that information is available on an OBD II generic scan tool that can be purchased for under $300. (Bob Pattengale, 2005)
References
Graham, Douglas B., How OBD II tests can be done on non-OBD II vehicles, Motor Age, Oct (1994), Vol. 113 Issue 10, p33
James V. Delong, Out of Bounds, out of Control: Regulatory Enforcement at the EPA, Cato Institute, (2002)
John Kirton, NAFTA's Trade-Environment Regime and Its Commission for Environmental Cooperation: Contributions and Challenges Ten Years, Canadian Journal of Regional Science, Vol. 25, (2002)
Bob Pattengale, Interpreting Generic Scan Data, Moto Magazine, (March 2005)