Computers and Control Systems: Description and Operation

DIAGNOSTIC TROUBLE CODES
Diagnostic Trouble codes, when set, indicate that the Powertrain Control Module (PCM) has detected a malfunction in a particular circuit or system. The PCM is programmed with routines or checks that it follows only under prescribed conditions (called Test Conditions). When these conditions exist, the PCM checks certain circuits or systems for a malfunction. These checks are called failure conditions. When the failure conditions are true, a diagnostic trouble code is set. Some diagnostic trouble codes alert the driver through the SERVICE ENGINE SOON Malfunction Indicator Lamp (MIL) or a message. Other codes don't trigger a driver warning. Refer to System Diagnosis / Procedures / Trouble Code Descriptions for a complete list of PCM diagnostic trouble codes and what driver alerts they trigger. The PCM also saves data and input parameters when most codes are set. For information on this feature, refer to System Diagnosis / Procedures / Vehicle On-Board Diagnostic / Diagnostic Modes / PCM Snapshot. Testing and Inspection

Current Code
When the PCM determines that the test conditions and failure conditions for a particular code are true, it sets that code as Current. All codes remain current at least until the next time the PCM finds the test conditions are true and the failure conditions are false. A few codes remain current until the next ignition cycle or until the failure conditions are false several times. These details are provided in the Diagnostic Trouble Code Charts.

History Code
After a code has been set, the PCM continues to monitor for the test and failure conditions. For most codes, if the test conditions are true and the failure conditions are false, the code is changed to history. Some codes require the test conditions to be true and the failure conditions false several times or require an ignition cycle to change from current to history. If there are no current codes for a period of 50 ignition cycles, all history codes will be cleared.

TROUBLE CODE DIAGNOSIS
This section uses diagnostic charts, wiring diagrams, and descriptive text to direct you in performing specific tests to locate and repair the problem. The diagnostic chart is a step by step procedure to determine the circuit or component that is the source of the problem. The wiring diagrams and text describing the system, test conditions, failure conditions, and actions taken when a code is set are located with the particular diagnostic chart. Refer to System Diagnosis / Procedures / Diagnostic Charts / Diagnostic Trouble Codes. Testing and Inspection