GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Chevrolet 🢒 1998 🢒 Chevy Suburban 4WD - RHD 🢒 DTC 🢒 DTC P1621 Control Module Long Term Memory Performance

Circuit Description

The VCM EEPROM contains data which is essential to running the engine and transmission. The VCM program stores this data in the EEPROM. This data remains in the EEPROM even if the battery is disconnected. At powerup, the VCM checks the integrity of this data. If the checksum of the EEPROM does not match a predetermined value, DTC P1621 sets.

Since this diagnostic pertains to an internal 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 VCM records the operating conditions at the time the diagnostic fails. The VCM stores this information in the scan tools Freeze Frame and/or the 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.

Test Description

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

  1. Replace the VCM even if this DTC exists only in history.

Step

Action

Value(s)

Yes

No

1

Important: Before clearing the DTCs, use the scan tool in order to record the Freeze Frame and the Failure Records for reference. This information will be lost when the Clear DTC Information function is used.

Was the Powertrain On-Board Diagnostic (OBD) System Check performed?

--

Go to Step 2

Go to Powertrain On Board Diagnostic (OBD) System Check

2

  1. Replace the VCM.
  2. Program the VCM. Refer to VCM Replacement/Programming .
  3. Perform the Crankshaft Variation Learn Procedure. Refer to Crankshaft Position System Variation Learn .

Is the action complete?

--

Go to Step 3

--

3
  1. Use the scan tool in order to select the DTC and the Clear DTC Information functions.
  2. Start the engine.
  3. Allow the engine to idle until normal operating temperature is reached.
  4. Select the DTC and the Clear DTC functions.
  5. Enter the DTC number which was set.
  6. Operate the vehicle within the conditions for setting this DTC as specified in the supporting text.

Does the scan tool indicate that this diagnostic ran and passed?

--

Go to Step 4

Go to Step 2

4

Use the scan tool in order to display the Capture Info and the Review Capture Info functions.

Are there any DTCs displayed that have not been diagnosed?

--

Go to the applicable DTC table

System OK