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Exhaust 101

On Board Diagnostics

WHAT IS OBD?

By the early 1980's, numerous vehicles were using electronics and on-board computers to control many of the engine's control systems, such as fuel and ignition. Sensors and actuators sense the operation of specific components (oxygen sensors) and actuate others (fuel injectors) to maintain optimal engine control. An on-board computer, known sometimes as a "powertrain control module" or an "engine control unit," controls all of these systems.

With proper computer software, the on-board computer is capable of monitoring all of the sensors and actuators to determine whether they are working as intended. It can detect a malfunction or deterioration of the various sensors and actuators, usually well before the driver becomes aware of the problem through a loss in vehicle performance or driveability.

Vehicle manufacturers had to develop ways to diagnose problems generated by the new electronic hardware found under the hood. The sensors and actuators, along with the diagnostic computer software in the on-board computer, make up what is called the On Board Diagnostic or OBD system.

The purpose of OBD was to monitor the many systems in the car. When the computer system of the car sees a fault, three things are supposed to happen.

• Set off a warning light on the dashboard to inform the driver that a problem existed.
• Set a code in the computer.
• Record the code in the computer's memory that can later be retrieved by a technician for diagnosis and repair.

CONNECTION BETWEEN OBD AND VEHICLE EMISSIONS

There are circumstances under which the vehicle computer will detect a system problem before the driver notices a driveability problem. Furthermore, OBD can detect problems that may not be noticeable upon visual inspection because many component failures that impact emissions can be electrical or even chemical in nature. By detecting these emission-related failures and alerting the driver to the need for potential repair, the EPA hopes that vehicles will be properly repaired before emissions become a problem.

This system worked so well that in 1986 California mandated that all cars sold in the state be equipped with OBD. This then became an industry standard throughout the nation, and all cars sold in the nation had some form of OBD.

OBD FIRST GENERATION - SHORTCOMINGS

This first version of OBD had a lot of shortcomings. First, it only covered the engine emission system. Another limitation of OBD was that it couldn't detect certain kinds of problems such as a dead catalytic converter or one that had been removed. Nor could it detect ignition misfires or evaporative emission problems.

Furthermore, OBD systems would only illuminate the light after a failure had occurred. It had no way of monitoring progressive deterioration of emissions-related components.

Additionally, there was no standardization throughout the industry. Each manufacturer had a different term for the warning light that was illuminated when a fault was determined.

• GM called it a "check engine" or "service engine" light.
• Chrysler called it a "power loss" light.
• Ford called it an "engine" light.
• Most foreign cars called it a "check engine" light.

This was not only confusing to the technician, but also to the motorist. Many motorists upon seeing the 'service engine' light illuminated, brought their car to a repair facility and either asked for an oil change or tune-up, expecting the light to go out. Needless to say, this did not happen, and after spending unnecessary money on service work, the system then had to be diagnosed and repaired. The coding system for each manufacturer was also different making diagnosis much tougher.

OBD II SYSTEMS

The Clean Air Act of 1990 mandated that, beginning with the 1996 model year, all vehicles less than 14,000 lbs. (e.g., passenger cars, pickup trucks, sport utility vehicles) will be equipped with a new version of on board diagnostics This system became known as OBD II. The manufacturers beat the deadline and almost all cars were equipped with OBD II in the 1995 model year.

The OBD II system monitors virtually every component that can affect the emission performance of the vehicle to ensure that the vehicle remains as clean as possible over its entire life, and assists repair technicians in diagnosing and fixing problems with the computerized engine controls. If a problem is detected, the OBD II system illuminates a warning lamp on the vehicle instrument panel to alert the driver. This warning lamp typically contains the phrase "Check Engine" or "Service Engine Soon".

The system will also store important information about the detected malfunction so that a repair technician can accurately find and fix the problem.

DIFFERENCES IN THE TWO SYSTEMS

What makes OBDII different from all the self-diagnostic systems that proceeded it is that OBDII is strictly emissions oriented. In other words, it will illuminate the Malfunction Indicator Lamp (MIL) anytime a vehicle's hydrocarbon (HC), carbon monoxide (CO), oxides of nitrogen (NOX) or evaporative emissions exceed 1.5 times the federal test procedure (FTP) standards for that model year of vehicle. That includes:

• Any random misfires causing an overall rise in HC emissions.
• Any time the operating efficiency of the catalytic converter drops below a certain threshold.
• Any time the system detects air leakage in the sealed fuel system.
• Any time a fault in the EGR system causes NO_x emissions to go up.
• Any time a key sensor or other emission control device fails.

In other words, the MIL light may come on even though the vehicle seems to be running normally and there are no real driveability problems.

The main purpose of the MIL lamp on an OBDII-equipped vehicle, therefore, is to alert motorists when their vehicles are polluting so they'll get their emission problems fixed. But as we all know, motorist are very good at ignoring warning lamps -- even when steam is belching from under the hood or the engine is making horrible noises. That's why regulators want to incorporate OBDII into existing and enhanced vehicle emissions inspection programs. If the MIL lamp is found to be on when a vehicle is tested, it doesn't pass even if its tailpipe emissions are within acceptable limits.

Don't think for a moment that OBDII is just a fancier version of self-diagnostic software. It's that and much, much more. OBDII-equipped vehicles typically have:

• Twice the number of oxygen sensors as non-OBDII vehicles (most of which are heated O₂ sensors). The additional O₂ sensors are located downstream of the catalytic converter.
• More powerful powertrain control modules, with either 16-bit (Chrysler) or 32-bit (Ford & GM) processors to handle up to 15,000 new calibration constants that were added by OBDII.
• Electronically Erasable Programmable Read Only Memory (EEPROM) chips that allows the PCM to be reprogrammed with revised or updated software changes using a terminal link or external computer.
• A modified evaporative emission control systems with a diagnostic switch for purge testing, or an enhanced EVAP system with a vent solenoid, fuel tank pressure sensor and diagnostic test fitting.
• More EGR systems with a linear EGR valve that is electronically operated and has a position sensor.
• Sequential fuel injection rather than multi-port or throttle body injection.

Among the many differences between OBD and OBD II, was the standardization of the system. All dashboard warning lights now say 'check engine', usually with a picture of an engine with the word 'check' across it.

The coding system is now standard. There are now over 400 possible trouble codes that can be stored in the system. If the gas cap is left loose and vapors are escaping from the gas tank, the 'check engine' light will be illuminated and a code will be set.

Benefit to the Environment

The intent of OBD systems is to assure proper emission system operation of each and every vehicle and light truck for its lifetime by monitoring emission-related components and systems for malfunction and/or deterioration. An important aspect of OBD is its ability to notify the driver of a problem before the vehicle's emissions have increased significantly. If the vehicle is taken to a repair shop in a timely fashion, it can be properly repaired before any significant emission increase occurs. OBD systems will also provide automobile manufacturers with valuable feedback from their customers' vehicles that can be used to improve vehicle and emission control system designs.

Benefits to the Consumer

OBD systems are designed to alert drivers when something in the emission control system begins to deteriorate or fails. Early diagnosis followed by timely repair can often prevent more costly repairs on both emission control systems and other vehicle systems that may affect vehicle performance such as fuel economy.

For example, a poorly performing spark plug can cause the engine to misfire, a condition sometimes unnoticed by the driver. This engine misfire can, in turn, quickly degrade the performance of the catalytic converter. With OBD detection of the engine misfire, the driver would be faced with a relatively inexpensive spark plug repair. However, without OBD detection, the driver could be faced with an expensive catalytic converter repair in addition to the spark plug repair.

Original manufacturers have an increased incentive to build a higher quality vehicle with better performance, reduced emissions, and more efficient powertrains to prevent problems that can lead to OBD detection. OBD systems will also provide far more information than ever before to help auto technicians diagnose and properly repair vehicles during their first visit to the repair shop, saving time and money for consumers.

Servicing OBD Related Problems

Only qualified, trained technicians equipped with the newest diagnostic and repair equipment should conduct OBD related service. All dealerships and independent repair shops should have qualified personnel for conducting OBD related repairs. Vehicle owners should ask their dealer and independent repair shops if their technicians have received proper training and have access to the necessary equipment to properly service OBD equipped vehicles.

Aftermarket Parts and OBD

Aftermarket parts will work with OBD systems. It is the responsibility of aftermarket parts manufacturers to ensure that their parts work properly with the vehicle for which they are designed. This is even more true for OBD. The OBD regulations have required manufacturers to devise technologies and monitoring strategies that didn't previously exist. However, EPA is confident that aftermarket part manufacturers who do a thorough job of replicating original equipment manufacturer parts and those who carefully develop specialty parts will be able to produce parts that work with the OBD system.