Automobile On-board Computers

Automobile on-board computers control engines, transmissions, brakes, traction and many other components.
These computers have several names and acronyms depending upon the manufacturer
and components they control. The most common name is PCM or Powertrain Control Modules.
Other examples are Body Control Modules (BCM), Transmission Control Modules (TCM),
Electronic Brake Control Module (EBCM) and Air Conditioning Control Module (ACM).
A variety of sensors, such as the oxygen sensor, throttle position sensor and manifold air temperature sensor, provide information to the on-board computer regarding the vehicle's engine operating
conditions.
Air Conditioning systems, vehicle air bags, and anti-lock brake systems also report to on-board computers.
On-board computers have a built in self testing system called self-diagnosis which means the on-board computer will monitor many or all of the vehicle's sensors and controlled devices for proper operation.

A diagnostic trouble code or DTC is detected and set when one of the monitored devices
is not functioning properly. This malfunction is stored into the on-board computer's memory as a DTC number that is related to a specific sensor or other problem.
The computer can later be accessed using the EASE Scan Tool or other scan tools and code reading devices to obtain the codes
stored in the on-board computer memory.

OBD II
Federal law required all vehicle manufacturers to meet On Board Diagnostics, Second Generation or OBD II standards by 1996.
In order to meet this standard, the automobile's on-board computer must
monitor and perform diagnostic tests on vehicle emissions to ensure that the vehicle is operating at an acceptable (legal) emission level. The maximum allowable emission level is set by
the Federal Test Procedure (FTP).

All 1996 and newer passenger vehicles are OBD II compliant. All OBD II vehicles have the same 16 pin diagnostic connector or DLC. This eliminates the need to have a manufacturer specific connector to connect to your vehicle. (Some 1994 and 1995 vehicles have this connector, however,
this does not mean that the vehicle is OBD II compliant. )

How to tell if the Vehicle is OBD II
An OBD II Scan Tool should be designed to work with all OBD II compliant vehicles.

•1996 or newer OBD II Compliant Vehicle (Includes All Domestic, Asian and European
Vehicles)
•Some 1994 and 1995 vehicles are OBD II Compliant
Exceptions: CNG (Compressed Natural Gas) vehicles and vehicles that are designated as Flex Fuel that are 1996 and newer may not be OBD II Compliant. Be sure to check the Vehicle Emission

Control Information Label.
For your vehicle to be OBD II compliant it must have a 16 pin DLC under the dash and the vehicle Emission Control Information label must state that the vehicle is OBD II compliant. This label is located on the inside of the hood on most vehicles.
Note the last line of the label states that the vehicle is OBD II certified.

DLC- Data Link Connector - a 16 position connector located under the driver side dash of most vehicles.
Vehicle Emission Control Information Label - located inside the hood of most vehicle. Use it to identify whether or not the vehicle is OBD II compliant.
OBD II Inspection and Maintenance (I/M) Readiness Monitors
A monitor is a piece of software in one of the vehicle's on-board computers that has the job of monitoring a specific piece of the engine.
There are two types of monitors: continuous and non-continuous.
A continuous monitor runs continuously during vehicle operation. A non-continuous
monitor requires enabling criteria to make it run. Some examples of enabling criteria are vehicle acceleration/deceleration to a certain speed, engine temperature and driving the vehicle at a certain speed for a period of time.
For OBD II vehicles, there is a fixed list of 11 monitors: 3 continuous and 8 non-continuous.
The 11 monitors are not applicable for all vehicles.
The Inspection & Maintenance screen of the EASE OBD II Scan Tool software lists the availability and status of your vehicles monitors.
In order to pass an emissions inspection all of the supported monitors must be completed.
The Continuous Monitors are:
•Misfire •Fuel System •Components

The Non-Continuous Monitors are:
•Catalyst •Secondary Air System •Oxygen Sensor
•Heated Catalyst •A/C System •Oxygen Sensor Heater
•Evaporative System •EGR System

Vehicle Emission Control Information

This vehicle is equipped with electronic
control systems . Engine idle speed, idle mixture and ignition timing are not adjustable.
This vehicle conforms to U.S. EPA and
California regulations applicable to 1998 model year new passenger cars certified for sale in California.
HSC 39037.05 Low Emission Motor Vehicle. OBD II Certified.

OBD II Certified
OBD II Diagnostic Trouble Codes or DTCs
OBD I malfunction code numbers varied between manufacturers, years, makes and models.
OBD II requires that all vehicle manufacturers use a common Diagnostic Trouble Code or DTC numbering System.
There is a generic DTC listing that all manufacturers must use. Since the generic listing was not specific enough, most manufacturers came up with their own DTC listing which are called manufacturer specific codes. Both generic and manufacturer specific codes are 5 digits.

The first digit is a letter which identifies the function of the device which has the fault. This digit can be either
P Powertrain
B Body
C Chassis
U Network or data link code

The second digit is either a 0 or 1 and indicates whether the code is generic or manufacturer specific.
0 Generic
1 Manufacturer Specific

The third digit represents the specific vehicle system that has the fault.
Listed below are the number identifiers for the powertrain system.
1 Fuel and Air Metering
2 Fuel and Air Metering (Injector Circuit Malfunctions Only)
3 Ignition System or Misfire
4 Auxiliary Emission Control
5 Vehicle Speed Control and Idle Control System
6 Computer and Auxiliary Outputs
7 Transmission
8 Transmission

The last two digits indicate the specific fault index.
On OBD II vehicles there are two different types of DTCs: Stored and Pending. For a DTC to become Stored, certain malfunction conditions must occur.
The condition(s) required to Store a
code are different for every DTC and vary by vehicle manufacturer. In order for some DTCs to become Stored, a malfunction condition has to happen more than once. If the malfunction conditions are required to occur more than once, the potential malfunction is called a Pending DTC.
The DTC remains Pending until the malfunction condition occurs the required number of times to make the code Stored. If the malfunction condition does not occur again after a set time the Pending DTC will
be cleared.

OBD II Freeze Frame Data
When the first emissions related powertrain DTC becomes stored, the PCM will capture (save) a block of current engine parameters. This list of parameters is called Freeze Frame Data and consists of a fixed list of parameters. For vehicles which do not support all parameters, only the applicable
ones are stored. The first parameter in the list is always the DTC that caused the Freeze Frame.
When DTCs are cleared, the Freeze Frame Data is cleared from the vehicle's PCM.
However, the freeze frame data is saved by the scan tool software until you reconnect to another vehicle or exit the program.

OBD II Malfunction Indicator Lamp (MIL)
The Malfunction Indicator Lamp (MIL) is located in the instrument panel on the dashboard and is either a red or yellow labeled lamp. The MIL is normally off and will illuminate if a system or component either fails or deteriorates to the point where the vehicle emissions could rise 1.5 times above the FTP set emissions level.

OBD I DTCs (Malfunction Codes)
On-board computers have a built in self testing system called self-diagnosis which means the on-board computer will test many or all of the vehicle's sensors and controlled devices for proper operation.
A malfunction is detected and a code is set when one of the monitored devices is not
functioning properly. This malfunction is stored into the on-board computer's memory as a code number that is related to a specific sensor or other problem.
The computer can later be accessed using a scan tool or other code reading devices to obtain the codes, (called diagnostic trouble codes
(DTCs), malfunction codes, trouble codes, or fault codes), stored in the on-board computer memory.
Malfunction code numbers and meanings vary from vehicle to vehicle and year to year even on the same vehicle model.

OBD I Check Engine Lamp
The Check Engine Lamp, also called the Service Engine Soon, and Malfunction Indicator Lamp (MIL) is located in the instrument panel on the dashboard and is either a red or yellow labeled lamp.
The Check Engine Lamp is normally off and is turned on by the on-board computer when the engine is running and a malfunction condition is being detected.
The lamp will stay lit as long as the
problem is present and a malfunction code will be stored in the on-board computer's memory.
If the engine starts operating under acceptable operating conditions again the Check Engine Lamp will be turned off by the computer, however, the code will remain stored in memory.
The lamp may be turned off and on several times while the engine is running if your vehicle has an intermittent problem.
After you see the Check Engine Lamp come on either steadily or intermittently while the
vehicle is running, scan the codes at your earliest convenience to obtain the malfunction code(s).

If your in tne need of scan tools for either your C4 or C5, we have tested several and can obtain a scanner via us that has passed out testing at a discounted price.
We also can obtain MAF translators for you.

This is a good post JR, My brother-in-law just had to finish another course for his smog license. He says wait till you see OBDIII which should be appearing around 2005. DTC's are transmitted via gps and if not corrected by a certain timeframe, could result in consequences. Not sure what tho... could be either fines or registration issues.


Bobby

Thanks Robert, yes your correct, and if people do not learn how to monitor and resolve OBD-II issues today, it will only get worse when they own OD-III based engine management cars.
You might ask your bother in law to give us a little heads up on what we should expect with OBD-III

John