The Hyperlogos

OBD-II (pronounced "Oh Bee Dee - Two") which stands for On-Board Diagnostics second revision, is a standard for automotive computer operation and diagnosis. It became mandatory in 1996 for all cars sold within the US to be equipped with an OBD-II system. Some USDM cars went OBD-II as early as 1994. Computer-controlled cars prior to OBD-II are referred to as OBD-I cars, though there was not actually any standard prior to OBD-II. In the U.S., OBD-II's main job is to make sure that vehicle emissions do not exceed 1.5 times the "FTP", or federal test procedure, which measures vehicle emissions. OBD-II provides a specification for what data must be collected, and how it should be presented to the technician.

OBD-I

Electronically-controlled engine systems previous to OBD-II are now retroactively referred to as OBD-I. So-called OBD-I vehicles followed no formal specification whatsoever and vary widely from manufacturer to manufacturer. Most systems of this era are primitive types which do not allow for reprogramming, but which can be monitored and often "tuned" (only temporarily, and only for the purposes of diagnosis) through a manufacturer-specific connector on the computer. During the OBD-I era this was commonly known as the "ECU" for "Engine Control Unit", but OBD-II formally refers to is as the "PCM" or "Powertrain Control Module".

In general, what all OBD-I systems have in common is a means of reading or estimating air flow, a means of controlling air flow at least to some degree, and a means of controlling the amount of fuel. In general, the user never controls the amount of fuel delivered in these systems, although in carbureted engines they may still operate an acceleration pump. An O² sensor determines whether the mixture should be richer or leaner during closed-loop operation; at wide-open throttle (WOT) the fuel delivery is determined by a "map" or table, which in some computers is updated over time based on sensor input. Nearly all of these systems also have some sort of solenoid or vacuum-operated valve which recirculates some exhaust gases and the crankcase vapors into the intake (especially at idle) to raise combustion temperatures and burn unused gases, and a sensor to detect if it is working.

OBD-II

While OBD-I systems dramatically reduced the quantity of pollutants produced by automobiles as compared to non-computer control systems like mechanical fuel injection or carburetors, the need to further reduce emissions led to more complicated systems. All OBD-II cars have at least two oxygen sensors, which detect the presence of O² in the engine exhaust. This voltage-generating sensor produces 450mV1 in the case of stoichiometric combustion — the state in which all of both the air and fuel are consumed in the reaction. In both OBD-I and OBD-II cars the PCM will be constantly running the car either richer if the sensor says the mixture is lean, or vice versa, but OBD-II cars also have an additional oxygen sensor behind the catalytic converter which allows them to determine if the catalyst is functioning.

The Malfunction Indicator Light

The MIL, or malfunction indicator light, is present to tell the driver that their vehicle may be producing emissions greater than 1.5 times the federal limit. However, because this will not happen if the powertrain is operating properly, it also serves to tell the driver when something is wrong with it. Most equipment failures will cause most OBD-II cars to detect an emissions problem, set a DTC (trouble code) and illuminate the MIL. Particularly serious failures that indicate that damage to the catalyst (and possibly the engine) is occurring will cause the MIL to flash two or more times per second, indicating that the vehicle should not be driven at any speed.

Vehicles may also have a CEL or "check engine light" as was common for OBD-I cars, but it is not required. The MIL differs from a "check engine light" in that there are specific conditions which cause it to light which are the same for all OBD-II cars.

Trips

The MIL being lit does not necessarily mean that there is a real problem, nor does the MIL not being lit mean that there is no problem. This is because OBD-II has implemented concepts called "monitors" and "trips" to control when a trouble code is set. A monitor is a computer program which tests the output of assorted sensors for validity. Some monitors require that the vehicle be driven in a certain way for the monitor to run; for example you must drive at freeway speeds and then slow without braking in order to run some monitors, like the one that checks the fuel evaporative system (the "EVAP" monitor.) A trip, then, is a complete set of driving conditions which allow a specific monitor to run.

Monitors

There are 8 monitors which all OBD-II vehicles are required to perform, and about 13 monitors in typical use. Three monitors are continuous monitors, which run constantly. An error detected by these monitors will immediately light the MIL, and these monitors' completion is an enabling criteria for the other monitors. During a "smog check" (emissions test) in some U.S.A. states (notably California) the smog test computer is connected to the PCM via the data link connector and if all of these monitors have not completed twice (with the exception of the continuous monitors which will always have run) or even have not passed then you can fail your smog check - even for something as foolish as not fastening your gas cap properly, although any competent shop will detect and correct this particular condition immediately2.

Continuous Monitors

The continuous monitors begin running as soon as the car is started up. They are the the misfire monitor, the comprehensive components monitor, and the fuel monitor. The misfire monitor watches the crankshaft position sensor (CKP) and when present, the camshaft sensor, for smooth output. Sufficient variance in the signal from this sensor can indicate a misfire. Some vehicles have problems with this monitor tripping during some driving conditions, such as driving over railroad tracks, that can cause sufficiently rapid changes in load to affect the output of these sensors.

The comprehensive monitor watches the output from all sensors to determine not only if the signal is within an acceptable range, but also whether the signal is reasonable given the state of other sensors and actuators on the vehicle. If the coolant temperature is 200 degrees, and the O2 sensor is not generating a voltage between 200 and 800mV, then the O2 sensor is probably nonfunctional. If one of these continuous monitors detects a fault, they cause the MIL to illuminate immediately. This type of fault is considered an "A"-type fault, as faults found in these monitors are considered capable of causing damage to the engine or catalytic converter, or of making the PCM produce invalid output.

The fuel systems monitor watches for changes in fuel system operation that could cause excessive emissions. OBD-II introduced a standard terminology for the amount of fuel added or removed from what the fuel map suggests the vehicle will need at a given time, called "[[fuel trim]]". (While highly appropriate, according to one OBD-II committee member, the word "trim" was located on a menu during one of their dinner engagements.) There are short- and long-term fuel trim adjustments. When the short-term fuel trim reaches one end or the other of its adjustment scale, the long-term fuel trim is adjusted in the same direction. If the short or long-term fuel trim goes too far out of specification this monitor will set a code and illuminate the MIL.

Other Mandatory Monitors

There are also other monitors which do not run continuously. The OBD-II mandated monitors in this category are for the oxygen sensor, oxygen sensor heater, catalyst efficiency, exhaust gas recirculation (EGR), and evaporative emissions. The first test attempts to make sure that the O2 sensor is operating properly, which it does partly by monitoring the other sensors on the vehicle, especially the coolant temperature sensor. The CTS is meanwhile being monitored by the (continuous) comprehensive components monitor. The second test is mandatory because all OBD-II vehicles have at least one O2 sensor downstream of the catalyst which is used for the third test, catalyst efficiency. These sensors may not be heated enough for operation by the exhaust gases, and in any case would take a long time to heat, so they contain a heating element to bring them up to operating temperature.

In order to reduce NOx production, the EGR system reduces combustion chamber temperatures by introducing inert gases - in this case exhaust gases which are primarily (you hope) composed of carbon dioxide. This is done only during cruising conditions as it would both reduce power output at wide open thottle and cause problems maintaining an idle. The EGR systems monitor checks the operation of the EGR system by watching other sensors and comparing them to what it expects from normal operation. The evaporative emissions monitors checks for fuel system evaporative airflow, and checks for leaks by doing pressure and/or vacuum tests. The evap monitor is required to detect air leaks of 0.040" orifice size or smaller.

Optional Monitors

Aside from these required monitors, there are also some optional monitors. Some of them are required in certain situations; for example vehicles with air conditioning which utilizes CFCs like R-12 must monitor the A/C system pressure to detect a loss of refrigerant — this is considered an optional monitor since you have the option to use a more environmentally friendly refrigerant, for example HFC-13a. There is a special monitor required on vehicles with a heated catalytic converter, which comes up to temperature before the vehicle is even started. There is a monitor for the PCV system which manufacturers may elect to implement, and there may also be monitors to test the operation of the secondary air conditioning system and the thermostat.

If any of these monitors detect a fault on two consecutive trips then they will set a B or C-type DTC. The type is significant because when a DTC is stored, a "freeze-frame" is captured that reflects the state of many of the sensors in the system, and a higher-priority fault overwrites the freeze frame data as only one freeze frame may be stored at a time. Any OBD-II compliant scan tool can extract this information. Some PCMs will store more than one "frame", but the manufacturer's scan tool is usually required to examine the extended data.

Manufacturers are motivated to implement optional monitors because they provide additional troubleshooting information which may be useful to the technician. Manufacturers have attempted to keep many of these codes to themselves (along with others related to tuning, upgrading, and other features of the typical OBD-II PCM) and legislation has been enacted to force them to share them for a "reasonable fee" (definition pending.)

OBD-II Data Link Connector

In order to get data from the PCM, some type of data interface is required. OBD-II refers to this as the data link connector (or DLC) and goes so far as to specify not only the shape and pinout of the connector, but the area in which it is installed on the vehicle, essentially in or around the driver's seat area. When made accessible it must be visible while sitting in the driver's seat, but it need not be obvious how to gain access to it in the first place, and it may be behind the ashtray, behind a trim panel, or even behind a vent.

OBD-II specifies a standard connector and pinout, but only three pins on the connector are mandatory: Two grounds and a battery voltage connection. Data connections can be made at any position on the connector. Data is provided at one of three baud rates and the specification does not require auto-baud control. Some cars have both the standard DLC, and the same signals on a different, proprietary connector. Non-manufacturer-specific PCM scan tools often use a small jumper board which snaps into the OBD-II cable connector to change the pinout to reflect the position of the data lines. While the assorted communications protocols used with OBD-II do have "standard" pins allocated on the OBD-II interface connector, it is not required by the OBD-II specification that the manufacturer not use the pin locations normally used by other protocols for their own purposes.

OBD-II connector pinout

.---------------------------------------.
\                                       /
 \    1   2   3   4   5   6   7   8    /
  \                                   /
   \  9  10  11  12  13  14  15  16  /
    \_______________________________/

Pins 4 and 5 are always grounds; chassis and signal grounds, repectively. Pin 16 is always battery power (which can be used to power the scan tool) and usually has its own fuse in the under-dash fuse block.

SAE standard J1850) provides for two data rates (10.4 kbps or 41.7 kbps) and uses either pulse width modulation (PWM) or variable pulse width modulation (VPW). J1850 VPW uses pins 2, 4, 5, and 16. J1850 PWM uses 2, 4, 5, 10 and 16. Most American cars use J1850.

ISO 9141-2 uses pins 4, 5, 7, 15 and 16. This is the most common type for non-US vehicles, especially Japanese models.

KWP2000 (ISO 14230) uses pins 4, 5, 7, 15 and 16.

CAN (Controller Area Network) uses 4, 5, 6, 14 and 16. CAN is gaining favor in Europe and is commonly seen in the US on imported big rigs (Volvo, Mercedes, etc.) CAN is also sometimes used between the PCM and the control computer for the transmission, but it's not on an OBD-II DLC there. CAN can reach speeds of several megabits per second (depending on the implementation.)

J1850 Pin Descriptions

2 J1850 Bus+
4 Chassis Ground
5 Signal Ground
6 CAN High (J-2284)
7 ISO 9141-2 K Line and ISO/DIS 14230-4
10 J1850 Bus
14 CAN Low (J-2284)
15 ISO 9141-2 L Line and ISO/DIS 14230-4
16 Battery power

Diagnostic Trouble Codes

OBD-II provides five digit alphanumeric trouble codes which begin with a letter followed by four numbers. The letter indicates the subsystem generating the trouble code, for instance P for powertrain. This is followed by either a 0 or 1, for OBD-II or manufacturer trouble codes respectively. The next digit tells us what part of the car the code is from, and the last two digits identify the specific fault. A car which has stored a diagnostic trouble code (or DTC) will light the malfunction indicator light MIL to alert the driver that there has been a problem.

Xnnnn First digit: Originating system
P: [[Powertrain]]
B: [[Body]]
C: [[Chassis]]
U: [[Network]]

xNnnn Second digit: OBD-II/OEM
0: OBD-II standard code
1: [[OEM]] code

xnNnn Third digit: Subsystem
Powertrain subsystems
1: Air/Fuel control
2: Fuel system
3: Ignition system
4: Auxiliary emissions control system
5: Idle/Speed control
6: Computer failures
7: Transmission

xnnNN: Fourth and Fifth digits: problem code

Legal Issues

Some controversy has arisen over OBD-II in that while there are standard codes which are common to all manufacturers, in fact hundreds of them, manufacturers also have their own codes which can be used through the OBD-II interface to do things like update the software in the PCM, change fuel map settings, and so on. Home mechanics and non-dealer mechanic shops would like this information to be mandatorily published by the manufacturers to assist them with automotive maintenance. More cars are soon to go "CAN" or controlled-area network, and you can already take a new OBD-II car to a dealer and have it reprogrammed for your local region to take elevation, average temperature and humidity, and other factors into account. It is not yet clear whether this system will be officially described as OBD-III. Manufacturers would prefer to keep control of their proprietary codes to bring service business to their dealerships - an important source of revenue.

• 1. For typical sensors; some types vary.
• 2. ...especially since the smog check in California includes a gas cap pressure test.