Diagnostic System Check
DIAGNOSTIC SYSTEM CHECKDESCRIPTION
The Diagnostic System Check-Computers and Control Systems is an organized approach to identifying a condition that is created by a malfunction in the powertrain control system. The Diagnostic System Check must be the starting point for any driveability concern. The Diagnostic System Check directs the service technician to the next logical step in order to diagnose the concern. Understanding and correctly using the diagnostic table reduces diagnostic time, and prevents the replacement of good parts.
DIAGNOSTIC AIDS
IMPORTANT: System voltage must remain between 9-16 volts for serial data communication.
Always store control module (Alternative Fuels ECM, VCM / PCM or ECM) DTC information into a scan tool before proceeding with diagnosis. The stored control module DTC information may contain data (Freeze Frame or Failure Records) useful for intermittent diagnosis. Clearing DTC information with a scan tool or the loss of control module power or ground will erase the control module DTC information. Some diagnostic procedures call for disconnection of control module connectors and loss of control module stored DTC information may occur.
TEST DESCRIPTION
Steps 1-10:
Steps 11-17:
The numbers below refer to the step numbers on the diagnostic table.
1. This step inspects for proper initial MIL operation.
2. If the scan tool fails to turn ON, check scan tool operation on a known good vehicle. The Data Link Connector (DLC) supplies the scan tool with a power circuit (terminal 16) and two ground circuits (terminals 4 and 5). All three circuits are required for proper scan tool operation.
4. Both control modules simultaneously test certain DTCs regardless of the fuel system that is in operation.
5. Checking for a shorted MIL circuit and proper control module operation of the MIL circuit.
7. The Fuel Indicator Lamp (FIL) located within the fuel gauge select switch button will illuminate when the engine is operating on gasoline.
The AFO Signal Command scan tool parameter will indicate the Alternative Fuels ECM's commanded state of the Alternative Fuel Operation circuit. The AFO Signal parameter will indicate the gasoline control modules monitored state of the Alternative Fuel Operation circuit. Regardless of the fuel system in operation these two data parameters will always indicate the same data unless an Alternative Fuel Operation circuit fault is present or the gasoline control module is programmed with standard gasoline software.
8. If actual and typical data values are different and a fault is suspected check the DTC information with a scan tool and verify that the suspected component or system DTCs have ran and passed. If they have passed use the sensor information in Description and Operation to help diagnose suspected sensor faults. If a system fault is suspected (EGR operation, EVAP operation, etc.) refer to the DTC that applies to the system and if possible use the diagnostic table to aid in diagnosis.
10. DTC P1665 may set if there is an AFO Signal circuit fault or if the PCM is programmed with standard gasoline software.
12. Check for applicable bulletins before proceeding with DTC diagnosis.
After determining which control module has stored DTCs, diagnose DTCs in the following order:
^ Control Module MEMORY or INTERNAL FAULT DTCs (PCM Memory, PCM Not Programmed, etc.)
^ SYSTEM and IGNITION VOLTAGE DTCs (System Voltage High or Low, etc.)
^ COMPONENT/ CIRCUIT DTCs (TP Sensor Circuit Voltage High or Low, Oxygen Sensor Circuit Insufficient Activity, etc.)
^ SYSTEM DTCs (MAP System Performance, TP Sensor Performance, etc.)