On-Board Diagnostics: Notes

• The job of diagnostics is to detect faults and store them so that they can later be read off using the diagnostic tool.
• Diagnostic trouble codes are the result of the fact that much of the software in modern control modules is constantly looking for faults. In an engine management system, for example, more than 50 % of the program code nowadays is used entirely for diagnostics.
• ISO/SAE has specified what diagnostic trouble codes should look like, which is a legal requirement in the USA. ISO/SAE determines all codes which have 0 in the second position.

Examples of diagnostic trouble codes: 

• P0107  Manifold Absolute Pressure/Barometric Pressure Circuit Low Input
• P0303  Cylinder 3 Misfire Detected
• P0601  Internal Control Module Memory Check Sum Error
• P0705  Transmission Range Sensor Circuit Malfunction (PRNDL Input)

The following are monitored:

• Internal control module faults
• Analog inputs
• Digital inputs
• Outputs