Diagnostic Circuit Check - With Diagnostic Scan Tool

"Scan" Diagnostic Circuit Check:

"SCAN" DIAGNOSTIC CIRCUIT CHECK

Circuit Description:

The purpose of the "SCAN" Diagnostic Circuit Check is to

1) make sure the "SERVICE ENGINE SOON" light works,

2) The ECM is operating and can recognize a fault, and

3) to determine if any codes are stored. This is the starting point for any diagnosis. It is important to understand the "SCAN" tool operation and limitations. See the introduction and general description.

The "Code Definitions", which immediately follows the "Diagnostic Circuit Check", will help to determine if the fault is still present. From there you will be directed to the code chart or see Diagnosis By Symptoms - No Trouble Codes Stored, which explains how to check for intermittents. In most cases, the facing page for a chart will also address the normal operation, expected values and provide some diagnostic aids.

Test Description:

1. Checks for the proper operation of the "SERVICE ENGINE SOON" light. With the key "ON," "SCAN" tool not connected, and the engine not running, the light should be on steady.

2. Grounding the test terminal will flash a Code 12 and any stored trouble codes. The light must go "ON" and "OFF" for a proper code. If the light goes from "Bright" to "Dim" this is not considered a code. See Chart A-6 in that case.

3. If the "SCAN" tool is not operating, try it on another vehicle. If it operates, then the cigar lighter socket should be checked for battery voltage and a good ground. If the "SCAN" tool displays "No Data" or "No ALCL,", with the ignition "ON", see chart A-6.

4. "NO CODES", at this point, indicates that the fault is intermittent and the "System Performance Check" should be performed. If a code or codes is displayed, code definitions starting on the page following this chart, should be consulted.