Diagnostic System Check - Vehicle

Circuit Description

The ignition mode switch has 2 contact buttons.

The upper button, with a circle indicator on it, that starts the engine if the brake pedal is applied.

The lower button has 2 indicators on it, O and ACC. This button cycles the ignition through the following modes:


OFF This is the normal state upon entering the vehicle, and can also be reached from any other mode by depressing the button. The O indicator illuminates.



Accessory This mode can be reached from the Off state by depressing the button. The ACC indicator illuminates.



ON with the engine OFF This state can be reached from the Off or Accessory modes by depressing the button for about 5-6 seconds. The upper LED illuminates.

Test Description

The numbers below refer to the step numbers on the diagnostic table.

  1. This step insures that the battery, and the vehicle primary power and ground systems are functioning correctly.
  1. Lack of communication may be due to a particular malfunction of a serial data circuit. The link to "Data Link References" will provide a list of modules and the associated data network no communication diagnostic link.






Perform the following preliminary inspections:


Ensure that the battery is fully charged. Refer to Battery Inspection/Test in Engine Electrical.



Ensure that the battery cables are clean and tight.



Inspect for any open fuses. Refer to Power Distribution Schematics and Electrical Center Identification Views .



Inspect the easily accessible systems or the visible system components for obvious damage or conditions that could cause the symptom.



Ensure that the grounds are clean, tight, and in the correct location. Refer to Ground Distribution Schematics and Power and Grounding Component Views .



Inspect for aftermarket devices that could affect the operation of the system. Refer to Checking Aftermarket Accessories in Wiring Systems.



Search for applicable service bulletins.

Did you find and correct the condition?

System OK

Go to Step 2


Install a scan tool.

Does the scan tool power up?

Go to Step 3

Go to Scan Tool Does Not Power Up in Computer/Integrating Systems


Turn ON the ignition, with the engine OFF.

Is the NO FOB DETECTED message displayed on the driver information center (DIC)?

Go to Key Fob Not Detected in Keyless Entry

Go to Step 4


  1. Turn ON the ignition, with the engine OFF.
  2. Attempt to establish communication with all of the control modules on the vehicle. Refer to Data Link References in Computer/Integrating Systems for the modules you should expect to observe.

Does the scan tool communicate with all of the expected vehicle control modules?

Go to Step 5

Go to Data Link References in Computer/Integrating Systems


Attempt to start the engine.

Does the engine crank?

Go to Step 6

Go to Symptoms - Engine Electrical in Engine Electrical


Attempt to start the engine.

Does the engine start and idle?

Go to Step 7

Go to Engine Cranks but Does Not Run in Engine Controls - 6.0L


Important: Do NOT clear any DTCs unless instructed by a diagnostic procedure.


Diagnose the DTCs in the order that the DTCs appear on the scan tool or mis-diagnosis may occur.



If multiple powertrain DTCs are stored, diagnose the DTCs in the following order:

  1. Component level DTCs, such as sensor DTCs, solenoid DTCs, and relay DTCs.
  2. System level DTCs, such as misfire DTCs, evaporative emission (EVAP) system DTCs, and fuel trim DTCs.

Advance to the List All DTCs screen on the scan tool.

Does the scan tool display any DTCs?

Go to Diagnostic Trouble Code (DTC) List - Vehicle

Go to Step 8


Is the customers concern with inspection/maintenance (I/M) testing?

Go to Inspection/Maintenance (I/M) System Check in Engine Controls - 6.0L

Go to Symptoms - Vehicle