OBD II Explained

What Is "OBD II"?

"OBD II" stands for On Board Diagnostics, level 2. OBD II is a federally mandated engine & emissions management standard for all passenger cars sold in 1996 and beyond. The NSX and some other cars switched to a fully ODB-II compliant system in 1995 (a year earlier than required by law). The certification is actually for the engine, not a particular vehicle model. The certification number for the NSX 3.0L engine (C30A) is SHN3.0VJGKEK.

From Road & Track


"This model year marks the first serious nationwide use of OBD II, short for On-Board Diagnostics, level 2. We in California have had it phasing in for a couple of years now; see Tech Tidbits, January 1994. What's more, GM, among others, had OBD II hardware aboard its cars last year, but only for company feedback, not customer/government information.


As their name suggests, On-Board Diagnostic systems use sensors, computational power and displays to inform the owner, mechanic, automaker and government (more or less, in that order) of powertrain problems. Of particular interest, of course, are those shortcomings affecting emissions controls (hence, the government's insistence on being in the loop).


Level 1 worried about things like really weird oxygen-sensor readings, catalytic-converter overheating and outright failure of one thing or another. OBD II upgrades this snooping to things like converter efficiency, engine misfire, even electrical integrity of idle air controllers, throttle-position sensors and transmission actuators.


For instance, we're all familiar with an oxygen sensor upstream of a catalytic converter. Its real-time analysis and the resulting dithering of fuel/air mixture ensure that the converter receives an optimal mixture of exhaust gases. But OBD II calls for another oxygen sensor downstream of the converter; this one, checking out that all the optimization went as desired.


Just between you and me, we should be so lucky to have this much quality control of government regulations.


Engine misfire is recognized by another neat OBD II feature. (And you can sense from my intellectual distance that, unlike automaker engineers, I didn't have to sweat through the immense complexity of these devices.) Any modern engine has a crankshaft sensor, its primary purpose related to ignition timing. But OBD II takes this sensor's input, analyzes it and recognizes the minute fluctuations that are concurrent with engine misfire.


If anything is amiss (and I mean anything!), diagnostic codes are stored in the car's computer memory, and a driver's warning is lit on the dashboard. Then begins a rather lengthy process of repair: The service specialist analyzes the coded diagnostic, fixes whatever's necessary, resets the light and, by way of certification, documents this incident to automaker and government.


What's more, there's a regulatory grabber: If more than 4 percent of some gizmo fails, this automatically invokes a recall of that particular engine family. Word among automakers is that OBD II may do more for assembly-line rigor, at least concerning emissions-control-related hardware, than all the touchie-feelie Quality Circles since the beginning of time."


From http://www.obdii.com

What is OBD-II?

On-Board Diagnostic systems are in most cars and light trucks on the road today. During the ’70s and early 1980’s manufacturers started using electronic means to control engine functions and diagnose engine problems. This was primarily to meet EPA emission standards. Through the years on-board diagnostic systems have become more sophisticated. OBD-II, a new standard introduced in the mid-’90s, provides almost complete engine control and also monitors parts of the chassis, body and accessory devices, as well as the diagnostic control network of the car.

Where’d it come from?

To combat its smog problem in the LA basin, the State of California started requiring emission control systems on 1966 model cars. The federal government extended these controls nationwide in 1968.

Congress passed the Clean Air Act in 1970 and established the Environmental Protection Agency (EPA). This started a series of graduated emission standards and requirements for maintenance of vehicles for extended periods of time. To meet these standards, manufacturers turned to electronically controlled fuel feed and ignition systems. Sensors measured engine performance and adjusted the systems to provide minimum pollution. These sensors were also accessed to provide early diagnostic assistance.

At first there were few standards and each manufacturer had their own systems and signals. In 1988, the Society of Automotive Engineers (SAE) set a standard connector plug and set of diagnostic test signals. The EPA adapted most of their standards from the SAE on-board diagnostic programs and recommendations. OBD-II is an expanded set of standards and practices developed by SAE and adopted by the EPA and CARB (California Air Resources Board) for implementation by January 1, 1996.

Why do we need it?

The Environmental Protection Agency has been charged with reducing "mobile emissions" from cars and trucks and given the power to require manufacturers to build cars which meet increasingly stiff emissions standards. The manufacturers must further maintain the emission standards of the cars for the useful life of the vehicle. OBD-II provides a universal inspection and diagnosis method to be sure the car is performing to OEM standards. While there is argument as to the exact standards and methodology employed, the fact is there is a need to reduce vehicle emitted pollution levels in our cities, and we have to live with these requirements.

Does my car have OBD-II?

All cars built since January 1, 1996 have OBD-II systems. Manufacturers started incorporating OBD-II in various models as early as 1994. Some early OBD-II cars were not 100% compliant. <<Click here> to see the dates OBD-II started being included on specific makes and models.

There are three basic OBD-II protocols in use, each with minor variations on the communication pattern between the on-board diagnostic computer and the scanner console or tool. While there have been some manufacturer changes between protocols in the past few years, as a rule of thumb, Chrysler products and all European and most Asian imports use ISO 9141 circuitry. GM cars and light trucks use SAE J1850 VPW (Variable Pulse Width Modulation), and Fords use SAE J1850 PWM (Pulse Width Modulation) communication patterns. <Click here> to see which cars use each system.

You may also tell which protocol is used on a specific automobile by examining the connector socket carefully. If the dash connector has a pin in the #7 position and no pin at #2 or #10, then the car has the ISO 9141 protocol. If no pin is present in the #7 position, the car uses an SAE protocol. If there are pins in positions #7 and #2 and/or #10, the car may use the ISO protocol.

While there are three OBD-II electrical connection protocols, the command set is fixed according to the SAE J1979 standard.

How do we measure OBD-II output?

Pre-OBD-II cars had connectors in various positions under the dashboard and under the hood. All OBD-II cars have a connector located in the passenger compartment easily accessible from the driver’s seat. Check under the dash or behind or near the ashtray. A cable is plugged into the OBD-II J1962connector and connected to a scan tool. This can range from a simple hand-held meter that provides a coded read-out of the various diagnostic functions, up to a large console computer-based unit costing thousands of dollars.

These large units are compatible with all cars and contain software that analyzes the signals received from the car, displays a text or diagrammed readout of any malfunctions found and suggests possible solutions to the problems.

Smaller units for the home or small shop technician can provide a variety of levels of data, some approaching the sophistication of the big shop consoles. However, they are usually limited to one OBD-II system type, unless adapters can be purchased.

What good does it do to measure OBD-II output?

OBD-II signals are most often sought in response to a "Check Engine Light" appearing on the dashboard or driveability problems experienced with the vehicle. The data provided by OBD-II can often pinpoint the specific component that has malfunctioned, saving substantial time and cost compared to guess-and-replace repairs. Scanning OBD-II signals can also provide valuable information on the condition of a used car purchase.

Tell me about that "Check Engine Light".

The service industry calls the Check Engine light on your dash an "MIL" or Malfunction Indicator Light. It shows three different types of signals. Occasional flashes show momentary malfunctions. It stays on if the problem is of a more serious nature, affecting the emissions output or safety of the vehicle. A constantly flashing MIL is a sign of a major problem which can cause serious damage if the engine is not stopped immediately. In all cases a "freeze frame" of all sensor readings at the time is recorded in the central computer of the vehicle.

Hard failure signals caused by serious problems will cause the MIL to stay on any time the car is running until the problem is repaired and the MIL reset. Intermittent failures cause the MIL to light momentarily and they often go out before the problem is located. The freeze frame of the car’s condition captured in the computer at the time of the malfunction can be very valuable in diagnosing these intermittent problems. However, in some cases if the car completes three driving cycles without a re-occurrence of the problem, the freeze frame will be erased.

OBD-II and your car’s health

Because of their investment in the equipment required, most repair shops charge a fee, some-times substantial, to attach the scanning equipment and diagnose problems using the OBD-II system signals. Home mechanics and small shop technicians have been restricted from working with these signals by the cost and technical complexity of the equipment. With the introduction of more economical and user friendly scanning devices, it is now practical for almost anyone to access OBD-II signals and use them for their own testing and repairs.

Scanners vary greatly in their complexity. The best connect easily and use software to quickly and automatically call up the OBD-II information. They should have recording ability so that data can be collected during a test drive without distracting the technician driving the car. A system connecting to a laptop or desk top computer provides expanded memory for data and the ability to export data to a spreadsheet or graphing utility.

Proprietary Sensor Readings

Though not part of the EPA’s OBD II standard, the diagnostic read-outs used by dealership technicians are also read through the OBD II connector. These service codes show you such things as knock sensor operation, FI pulse width, ignition voltage, individual cylinder misfires, transmission shift points and ABS brake condition. There can be over 300 readings available, depending on the vehicle manufacturer and model. Vehicles vary in the readings they will support. Scanners vary widely in the number of these signals that they can read. Some show just the basic OBD or OBD II signals, others show the full range of service codes.

OBD-II and performance tuning

While the vast number of drivers want nothing more than dependable, economical transportation, many of us are looking to OBD-II for extra performance. Earlier on-board computer systems had chips that could be replaced to adjust engine parameters for extra speed and power. While the OBD-II systems are sealed and do not allow chip replacement, they do provide a real time data acquisition system that is useful to tuners.

Power loaders can actually reprogram the performance parameters of the OBD-II system to accommodate performance options. At the current time the number of models they can service is limited, but the range is being extended. Be sure the person doing your reprogramming keeps the car in compliance with EPA emission standards. As aftermarket manufacturers develop additional solutions, we will add their information to our links.