Introduction

Self Diagnostic System Chart:

PURPOSE
The self-diagnostic system is designed to help technicians fix cars faster. This is done by having Engine Control Module (ECM) monitor certain input/outputs and store a trouble code 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" light 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" lamp 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" will illuminate only if trouble is found in certain circuits.


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 Oxygen Sensor operation.

TECH TIPS
On Board Diagnostics (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. What components and circuits are monitored. That means a loose Mass Air Flow Sensor (MAF) sensor connector can store the same code as the MAF sensor 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 Nissan OBD will store trouble codes:

^ Vehicle Speed Sensor Code (VSS): will be stored if the driver shuts off the ignition switch just before vehicle stops rolling. There is nothing wrong with the VSS.
^ Exhaust Gas Recirculating (EGR) Valve Code: will be stored if the EGR Temp. Sensor is coated with carbon and is not producing the output the ECM is expecting. EGR Valve OK, problem is the EGR Temp Sensor.
^ MAF Sensor 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.