Diagnostic Circuit Check
The Diagnostic Circuit Check ensures "CHECK ENGINE" light works, ECM is operating and can recognize a fault, and determines if any trouble codes are stored. Memory is cleared on the DEC system any time ignition switch is turned off. Therefore, unless the engine is left running when vehicle is brought in for complaint or code is reset during the diagnostic circuit check, the problem should be considered intermittent. This is the starting point for any diagnosis.
NOTE:
The following step numbers refer to the numbers in the accompanying flow chart(s).
- To check operation of "CHECK ENGINE" light, turn key to "ON", engine not running. The light should be on steady.
- Ground the "Test" terminal. This will flash a Code 12 and any stored trouble codes. The light must go on and off for a proper code. If light goes from bright to dim, this is not considered a code. See Chart A1 "CHECK ENGINE" LIGHT (CEL) MALFUNCTION in this case.
- This step determines if fault is still present or was intermittent and is no longer there. Memory is cleared when ignition is cycled off and engine is run for 30 seconds to see if trouble code(s) will reset.
- If light is on, fault is still present. Go to applicable trouble code chart.
- If light is off, fault is either intermittent, or is a code that cannot be set in the stall (24 or 41). For codes that cannot be set in the stall during the Diagnostic Circuit Check, the vehicle may be run on hoist or road tested.