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For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2005 🢒 TopKick C-Series (Conventional) C6/C7/C8 🢒 Engine 🢒 Engine Controls - 7.8L 🢒 DTC P1646

Circuit Description

The engine control module (ECM) supplies 5 volts reference voltage through the reference circuit 3 to the accelerator pedal position (APP) sensor. The ECM monitors the voltage on the 5-volt reference 3 circuit. If the ECM detects that the voltage is out of tolerance, this DTC will set.

DTC Descriptor

This diagnostic procedure supports the following DTC:

DTC P1646 5-Volt Reference 3 Circuit

Conditions for Running the DTC
    • The ignition is ON.
    • The battery voltage is between 10-16 volts.

Conditions for Setting the DTC

The 5-volt reference circuit 3 voltage is more than 5.5 volts or less than 4.5 volts.

Action Taken When the DTC Sets
    • The ECM illuminates the malfunction indicator lamp (MIL) when the diagnostic runs and fails
    • The ECM records the operating conditions at the time the diagnostic fails. The ECM stores this information in the Failure Records.

Condition for Clearing the DTC
    • A current DTC, Last Test Failed, clears when the diagnostic runs and passes.
    • Clear the MIL and the DTC with a scan tool.

Step

Action

Values

Yes

No

Schematic Reference: Engine Controls Schematics

1

Did you perform the Diagnostic System Check - Vehicle?

--

Go to Step 2

Go to Diagnostic System Check - Vehicle

2
  1. Observe the Failure Records data for this DTC.
  2. Turn OFF the ignition for 30 seconds.
  3. Start the engine.
  4. Operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions that you observed from the Failure Records.

Did the DTC fail this ignition?

--

Go to Step 3

Go to Intermittent Conditions

3
  1. Turn OFF the ignition.
  2. Disconnect the accelerator pedal position (APP) sensor connector.
  3. Turn ON the ignition, with the engine OFF.
  4. Observe the APP Sensor 3 parameter with a scan tool.

Is the APP Sensor 3 parameter within the specified value?

4.5-5.5 V

Go to Step 5

Go to Step 4

4
  1. Test the signal circuit between the engine control module (ECM) and the APP Sensor 3 for the following conditions:
    2. • A short to ground circuit
    • A short to battery voltage
  2. Repair the circuits as necessary.

Did you find and correct the condition?

--

Go to Step 8

Go to Step 6

5

Replace the APP sensor. Refer to Accelerator Pedal Position Sensor Replacement .

Did you complete the replacement?

--

Go to Step 8

--

6
  1. Turn OFF the ignition.
  2. Disconnect the ECM.
  3. Inspect for connections at the harness connector of the ECM.
  4. Repair the connections as necessary

Did you find and correct the condition?

--

Go to Step 8

Go to Step 7

7

Replace the ECM. Refer to Engine Control Module Programming and Setup , Engine Control Module Replacement/Restore Fuel Rate Data , and Fuel Injector Flow Rate Programming .

Did you complete the replacement?

--

Go to Step 8

--

8
  1. Clear the DTCs with a scan tool.
  2. Turn OFF the ignition for 30 seconds.
  3. Start the engine.
  4. Operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions that you observed from the Failure Records.

Did the DTC fail this ignition?

--

Go to Step 2

Go to Step 9

9

Observe the Capture Info with a scan tool.

Are there any DTCs that have not been diagnosed?

--

Go to Diagnostic Trouble Code (DTC) List - Vehicle

System OK