Diagnostic Circuit Check

Diagnostic Circuit Check determines if: 1) the "CHECK ENGINE" light works, 2) the ECM is operating and can recognize a fault, and 3) any codes are stored. It also checks to see if stored codes indicate an intermittent problem. This is the starting point for any diagnosis. If no codes are indicated, go to the System Performance Check. If no additional checks are called out from the System Performance Check, go to the Driveability Symptoms.

Check operation of the "CHECK ENGINE" light. Key "ON", engine not running, light should be on steady.
Grounding test terminal will flash a Code 12 and any stored trouble codes. The light must go "ON" and "OFF" to indicate a code. The light going from "Bright" to "Dim" is not considered a code. See CHART A-6.
This step will determine if any codes, other than Code 12, are still present or were intermittent and are no longer stored. Clear memory. Run vehicle for 2 minutes. See if trouble code(s) reset.
If the light is "ON", fault is still present. Go to the applicable trouble code chart. Refer to the appropriate TESTS/CODES article in this section. Press the light pen to the lower right corner "F10" to exit to the section menu. Locate the appropriate TESTS/CODES article. Press the light pen to the title and then scroll to find the test associated with the code.
If the light is "OFF", the fault is either intermittent, or it is a code that cannot be set with vehicle stationary. For codes that cannot be set during the Diagnostic Circuit Check, the applicable trouble code chart will determine if those codes are intermittent.

NOTE: The following flow chart may refer to test steps that are found at the end of this article or in other articles in this ENGINE PERFORMANCE section. Refer to appropriate TESTS W/ CODES, SYSTEM & COMPONENT TESTS, or TESTS W/O CODES articles in this section.

Fig 1: Diagnostic Circuit Check

(* ) - To clear codes, remove power to "R" terminal of ECM for 10 seconds by removing ECM connector with ignition OFF or ECM/BAT fuse, or disconnect ECM power feed at battery. Verify codes have been cleared. The System Performance Check should be performed after any repairs to the "System" have been made.

(** ) - It is possible to set a false Code 42 on starting, but the "Check Engine" light will not be ON. No corrective action is necessary.