On-Board Diagnostics

On-Board Diagnostic Facility (OBD I)
The ECM includes a fault diagnosis facility that continuously monitors the operation of the engine management sensors and components. If a fault is detected, the OBD system will activate the Malfunction Indicator Lamp (MIL) (CHECK ENGINE) warning in the instrument pack and flag a diagnostic trouble code (DTC) in the ECM memory. The ECM can be interrogated through the instrument pack LCD display (XJ6 Sedan Range) or center console LCD display (XJS Range). To display the code, switch the ignition OFF, wait 5 seconds, then switch the ignition ON (do not crank the engine). On Sedan Range vehicles, press the VCM button. On XJS Range vehicles, a code is automatically displayed 5 seconds after the ignition is switched ON. The CHECK ENGINE warning will display and the DTC will appear 5 seconds later. If two or more DTCs are flagged in memory, only the highest priority code will be displayed. If the vehicle battery is disconnected, all stored codes will be cleared from the ECM's memory. DTCs can also be accessed via serial communication





Limp Home Default
In order to allow vehicle operation if a malfunction occurs, "limp home" default values are incorporated as an ECM facility. If a sensor fault is detected by the OBD system, the ECM will substitute a nominal value for the missing input(s) as shown above.





Diagnostic Trouble Code Summary (listed in order of priority)