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XI. AUTOMOBILE ON-BOARD COMPUTERS AND DIAGNOSTIC TROUBLE CODES
The following section provides information on
1. OBD II
2. Automobile on-board computers
3. The Malfunction Indicator Lamp (MIL)
4. Freeze Frame Data
5. Diagnostic Trouble Codes (DTC)
6. Inspection and Maintenance (I/M) Readiness Monitors
1.0 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. )
2.0 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 software or other scan
tools and code reading devices to obtain the codes stored in the on-board computer memory.
3.0 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.
4.0 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. When
DTCs are cleared, the Freeze Frame Data is cleared from the vehicle’s PCM.
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