Reading and Clearing Diagnostic Trouble Codes

A diagnostic test procedure is a systematic method of checking the function of a vehicle control system to determine if it is working properly. The diagnostic test procedure is also a systematic method of determining where the problem is.

GUIDELINES
1. Begin diagnostics at the first test of the appropriate section.
2. Ensure that the correct response is provided for each section.
3. Before beginning any diagnostics, the battery must be fully charged.
4. At the completion of each repair reconnect any components, connectors or hoses that were disconnected for diagnostics.
5. Visually inspect the electrical connectors related to the circuit being diagnosed.
6. When using a jumper wire, ensure that either the jumper or the circuit is fuse protected.
7. Never substitute a test lamp for a voltmeter.
8. To protect the ECU (Engine Control Unit), do not allow a secondary ignition spark to exceed 1/4 inch.
9. Since MMC vehicles use an airflow meter, it is critical that all intake hoses are secure and in place.
10. Use extreme caution whenever it is necessary to remove a fuel line or filling. The fuel is under pressure and may cause personal injury.
11. If the vehicle has more that one fault code, begin diagnostics at the smallest code number.

ON-BOARD DIAGNOSTIC SYSTEM OPERATION
The Engine Control Unit (ECU) monitors input and output circuits within the fuel injection system. Some circuits are checked continually, others are only checked under certain conditions (starting, etc.). If the OBD system determines that one of the circuits is malfunctioning, a fault code will be stored in ECU memory. Each circuit has its own fault number and message.

HOW TO ERASE FAULT CODES
To erase fault codes, select "Erase Faults" on the DRBII and follow the instructions displayed.

THE CHECK ENGINE LAMP
if a critical fault code is set, the electronic control unit will turn on the "check engine" warning lamp and substitute a value for the failed sensor or component. All fault codes, except the speed sensor, are considered critical.