Federal Model (49-State)

PURPOSE
The On-Board Diagnostic (OBD) System is designed to help technicians fix cars faster. This is done by having the Engine Control Module (ECM) monitor certain input/outputs and store a Diagnostic Trouble Code (DTC) if the ECM detects a problem.

OPERATION
The self-diagnostic system has two modes. Mode 1 is the normal operating mode for the vehicle and will turn on the MALFUNCTION INDICATOR LAMP (MIL) if a monitored system has a fault. Mode 2 is used for self diagnostics. Both modes do different things depending whether the engine is running or not.

Mode 1:
Key ON Engine OFF:
- This is the MALFUNCTION INDICATOR LAMP (MIL) self-check. If the lamp does not illuminate in this mode check the electrical circuit.
Engine Running:
- This is the normal operating mode. The MALFUNCTION INDICATOR LAMP (MIL) will illuminate only if trouble is found in certain circuits.

Diagnostic Trouble Code (DTC) Table:

Mode 2:
Key ON Engine OFF:
- This is the stored codes self-diagnostic mode. Trouble codes will be extracted in this mode.
Engine Running:
- When the engine is running it is used to monitor the air/fuel mixture and O2 sensor operation.

TECH TIPS
OBD can help you fix cars faster if you follow a well thought-out game plan. Remember these rules:

- Do a thorough under-hood inspection first. Always start with "basics"
- Extract all codes, write them down. Erase the codes and test drive the car. See if the problem or the code(s) come back.
- Know WHY codes are stored. Components AND circuits are monitored. That means a loose Mass Air Flow Sensor (MAFS) connector can store the same code as the MAFS itself.
- Generally, the ECM is looking for either an open or a short before it will store a hard-part code.

Here are some known reasons that the OBD will store trouble codes:

- Vehicle Speed Sensor (VSS) Code: will be stored if the driver shuts off the ignition switch just before vehicle stops rolling. There is nothing wrong with the VSS).
- MAFS Code: will be stored from a loose connector, a poor ground wire in the harness, or even a small amount of water going through the sensor.