GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2000 🢒 GMC C Pickup - 2WD 🢒 DTC 🢒 DTC P1621 Control Module Long Term Memory Performance

Since this diagnostic pertains to an internal vehicle control module (VCM) diagnostic check, no wiring schematic is required.

Conditions for Running the DTC

The engine is cranking or running.

Conditions for Setting the DTC

The VCM is unable to correctly read data from the EEPROM memory.

Action Taken When the DTC Sets
    • The control module stores the DTC in history after the first failure but will not illuminate the malfunction indicator lamp (MIL).
    • The control module records the operating conditions at the time the diagnostic fails. The control module stores the failure information in the scan tools Freeze Frame/Failure Records.

Conditions for Clearing the MIL/DTC
    • A history DTC will clear if no fault conditions have been detected for 40 warm-up cycles.
    • A warm-up cycle occurs when the coolant temperature has risen 22°C (40°F) from the startup coolant temperature and the engine coolant temperature exceeds 70°C (160°F) during the same ignition cycle.
    • Use the scan tool Clear Information function.

Diagnostic Aids

Replace the VCM even if this DTC exists only in history. Refer to VCM Replacement/Programming .

Test Description

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

  1. This test checks if communication can be established between the scan tool and the VTD.

    2.

  2. This test checks for any opens in the serial data circuit between the VCM and the VTD. This also checks for any intermittent malfunctions associated with the serial data circuit.

    3.

  3. This step is to determine if the condition that set the DTC is still present. The engine must be cranked in order to set this DTC. If the condition is present, the engine will start and then stall, indicating the VCM is disabling fuel.

    4.

  4. Use a scan tool in order to clear and check for any DTCs in all the modules connected to the Class 2 Serial Data circuit (VCM, VTD, ATC, and BCM).

Step

Action

Value(s)

Yes

No

1

Did you perform the Powertrain On-Board Diagnostic (OBD) System Check?

--

Go to Step 2

Go to Powertrain On Board Diagnostic (OBD) System Check

2

  1. Install the scan tool.
  2. Turn ON the ignition, leaving the engine OFF.
  3. Attempt to establish communications with the VTD (Passlock) Control Module.

Does the scan tool communicate with the VTD (Passlock) Control Module?

--

Go to Step 3

Go to Diagnostic System Check - Theft Deterrent in Theft Deterrent

3

Check the Class 2 Serial Data circuit for the following intermittent conditions:

    • An open in the Class 2 Serial Data circuit between the VCM and the VTD (Passlock) Control Module
    • A short to ground
    • A short to voltage
    • Loose or damaged terminals at the data link connector (DLC), VCM or the VTD (Passlock) Control Module

Did you find a problem?

--

Go to Step 6

Go to Step 4

4

  1. Turn OFF the ignition.
  2. Connect or install any connectors or components that were disconnected or removed.
  3. Clear all DTCs using a scan tool.
  4. Turn OFF the ignition for 15 seconds.
  5. Attempt to start the engine.

Does DTC P1626 reset?

--

Go to Diagnostic System Check - Theft Deterrent in Theft Deterrent

Go to Step 5

5

This DTC is intermittent.

Are any additional DTCs stored?

--

Go to the applicable DTC table

Go to Diagnostic Aids

6

Repair the circuit as necessary.

Refer to Wiring Repairs or Connector Repairs in Wiring Systems.

Is the action complete?

--

Go to Step 7

--

7

  1. Turn OFF the ignition.
  2. Connect or install any connectors or components that were disconnected or removed.
  3. Clear all DTCs using a scan tool.
  4. Turn OFF the ignition for 15 seconds.
  5. Start the engine and allow it to run.

Are any new DTCs displayed?

--

Go to Powertrain On Board Diagnostic (OBD) System Check

System OK