Introduction

OBD is the abbreviation for On Board Diagnostics, a diagnostic system installed in the vehicle (on board). This diagnostic system is integrated in the Engine Control Module (DME 5.2) which constantly monitors certain emission related components of the vehicle. If a malfunction occurs, it will be indicated to the driver by the (MIL - Malfunction Indicator Lamp). At the same time, the malfunction will be stored in the memory of the Engine Control Module which can be read using a generic diagnostic scan tool.

The On Board Diagnostic II (OBD II) is the second generation of diagnosable engine management systems and is required by law beginning with model year 1996 for Porsche in USA.

OBD II includes the following requirements:
- A standardized diagnostic connector (DLC - Data Link Connector) in the area of the driver's foot well
- Standardized trouble codes for all manufacturers.(SAE J2012)
- The fault display (standardized Diagnostic Trouble Codes) via a generic diagnostic scan tool according to SAE J1978 (or Porsche scan tool 9288).
- Indication of operating conditions (freeze frames) during which a malfunction occurred.
- Requirement as to when and how an emission related malfunction is to be displayed.
- Standardized nomenclature and abbreviations of components and systems (SAE J1930).

Requirements for the OBD II System:
- Testing of the Three-Way Catalytic Converter efficiency.
- Recognition of combustion misfire
- Monitoring of the tank ventilation system.
- Monitoring of the secondary air injection.
- Monitoring of the adaptation limitation of the oxygen sensor system.
- Monitoring of the oxygen sensors.
- Monitoring of other emission relevant components (up to now OBD I component).
- Triggering the Malfunction Indicator Lamp (MIL) and fault memory.
- Protection against manipulation of control modules.
- Indication of the monitoring readiness (ready code).
- Indication of the Diagnostic Trouble Code.
- Storing of operation parameters in the event of a fault (freeze frame).
- Functional evidence of the OBD-system (trip, warm-up cycle, driving cycle).
- Standardized control module tester (Scan Tool).
- Standardized read out of operating data such as rpm, temperature, etc.

Operating conditions "Freeze frame"
For every malfunction, the ambient (operating) conditions (freeze frame) must also be stored. These are:
- Engine load
- Engine temperature
- Engine rpm
- Intake air temperature
- Voltage supply
- Throttle angle
- Tank level
- Counter for hours of operation

ECM signal output:

If a malfunction is detected and stored, these values are also stored and can be read out later with the scan tool. This makes troubleshooting much easier because operation data are available that were apparent at the moment of the malfunction.

These "freeze frame" data of a malfunction are stored at the first occurrence of the malfunction. If the same malfunction occurs a second time also these "freeze frame" data are stored. Each additional occurrence of the malfunction triggers the "freeze frame" data of the last malfunction.

This means that the "freeze frame" data can be read for the first occurrence of a malfunction and the last occurrence. They can be read out only with the scan tool.

DME-Engine Control Module signals
The Engine Control Module has to monitor the following signals (SAE J1979):
- Diagnostic Trouble Codes
- Engine load
- Engine coolant temperature
- Oxygen sensing status (oxygen sensor)
- Oxygen sensing-adaptation values
- Fuel pressure (when available)
- Ignition angle
- Intake air temperature
- Intake manifold pressure (when available)
- Air mass value
- Engine rpm
- Throttle position
- Vehicle speed

Standardized trouble codes SAE J 2012 (DTC - Diagnostic Trouble Codes)
Diagnostic Trouble Codes that are monitored by the engine control module are standardized, which means that all manufacturers must use the same Diagnostic Trouble Codes.

The Diagnostic Trouble Code (DTC) is always a 5-digit alphanumeric value, example "P0100)".

All P0xxx - codes are standardized codes. However each manufacturer can use other DTCs in addition to the standardized codes. This applies when the manufacturer integrated additional functions in the control module that can be diagnosed and that exceed the law. These codes are identified as "P1xxx", for example "P1100)".

The first digit of a code (letter) identifies the system that the code has set. There are 4 systems:

- B for body
- C drive train
- P for engine
- U for future systems.

For OBD II only the P - Code is required.

The second digit identifies the generic code (P0xxx), or the manufacturer code (P1xxx).

The third digit identifies the major subassembly where a malfunction occurred These are:
- P01xx or Fuel
- P02xx and air ratio
- P03xx Ignition system
- P04xx Additional emission controls
- P05xx vehicle speed and idle speed control
- P06xx Control module and initiating signals
- P07xx Transmission

The fourth and fifth digit is the identification of the subassembly or System.