General System Description

OVERVIEW
The On Board Diagnostic II (OBD II) system meets government regulations by monitoring the emission control system. When a system or component exceeds emission thresholds or a component operates outside of tolerance, a Diagnostic Trouble Code (DTC) will be stored and the Malfunction Indicator Lamp (MIL) will be illuminated.

OBD II vehicles are equipped with the following common features:

- Diagnostic connector
- Diagnostic input/output data
- Diagnostic procedures
- Terminology
- New emission related sensors
- Additional Heated Oxygen Sensors (HO2S) to determine efficiency of the catalytic converters.

Fault detection strategy and MIL operation are associated with Trips (specific instructions for vehicle operation within a period of time) and Drive Cycles, refer to Trips and Drive Cycles.

Each monitor (a test run by the engine control system), has requirements for setting and clearing DTCs and for controlling the MIL, refer to On Board Diagnostics System Monitors. On Board Diagnostics System Monitors

FREEZE FRAME DATA
Freeze Frame Data is a snap-shot of operating conditions at the time a pending code is triggered. When a set of freeze frame data is stored, this data remains in memory even if another emission-related DTC is stored, with the exception of Misfire Detection or Fuel System DTCs (which take priority).

The PCM confirms that a valid component or system failure is recorded by double checking a component or system before lighting the MIL. Freeze Frame Data is used to ensure that the failure has been seen under similar operating conditions. This data is accessible by a scan tool.