GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Chevrolet 🢒 2002 🢒 Express 🢒 DTC 🢒 DTC P1641

Circuit Description

A dash light is illuminated by the powertrain control module (PCM) when the ignition is ON with the engine OFF, or if the diagnostics have detected certain errors related to the engine performance or the engine sensor status. When the PCM is commanding the malfunction indicator lamp (MIL) ON, the voltage potential of the circuit will be low, near 0 volts. When the PCM is commanding the MIL OFF, the voltage potential of the circuit will be high, near the battery voltage. The primary function of the PCM in this circuit is to supply the ground for the MIL circuit.

Conditions for Running the DTC

The ignition is in the ON position.

Conditions for Setting the DTC
    • The MIL is requested ON.
    • The voltage on the MIL control circuit is high (near the battery voltage).

or

    • The MIL is requested OFF.
    • The voltage on the MIL control circuit is low, near 0 volts.

Action Taken When the DTC Sets
    • The control module stores the DTC information into memory when the diagnostic runs and fails.
    • The malfunction indicator lamp (MIL) will not illuminate.
    • The control module records the operating conditions at the time the diagnostic fails. The control module stores this information in the Failure Records.
    • The driver information center, if equipped, may display a message.

Conditions for Clearing the DTC
    • A current DTC Last Test Failed clears when the diagnostic runs and passes.
    • A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other non-emission related diagnostic.
    • Clear the DTC with a scan tool.

Diagnostic Aids

A malfunctioning bulb, the control circuit shorted to ground or battery voltage, will cause a DTC P1641 to set.

An intermittent may be caused by the following conditions:

    • Poor connections
    • Rubbed through wire insulation
    • Broken wire inside the insulation

Test Description

The number below refers to the step number on the diagnostic table.

  1. This test ensures that both the ON and the OFF states are commanded.

Step

Action

Yes

No

Schematic Reference: Engine Controls Schematics

1

Important: Before clearing the DTCs, use the scan tool Capture Info to save the Freeze Frame/Failure Records for reference. The data in the control module is deleted once the Clear Info function is used.

Did you perform the Diagnostic System Check-Engine Controls?

Go to Step 2

Go to Diagnostic System Check - Engine Controls

2

  1. Turn ON the ignition, with the engine OFF.

    2. Important: Ensure that both the ON and the OFF states are commanded. Repeat the commands as many times as necessary.

  2. Use a scan tool in order to command the lamp ON and OFF.

Does the lamp turn ON and OFF with each command?

Go to Step 3

Go to Step 4

3

The DTC is intermittent. If no additional DTCs are stored, refer to Diagnostic Aids. If any additional DTCs are stored, refer to those tables.

Are any additional DTCs stored?

Go to Diagnostic Trouble Code (DTC) List

Go to Diagnostic Aids

4
  1. Turn OFF the ignition.
  2. Disconnect the PCM connector containing the lamp control circuit.
  3. Turn ON the ignition, with the engine OFF.

Does the lamp illuminate?

Go to Step  5

Go to Step 7

5
  1. Turn ON the ignition, with the engine OFF.
  2. With a fused jumper wire connected to ground, probe the lamp control circuit in the PCM harness connector.

Does the lamp illuminate?

Go to Step 6

Go to Step 8

6
  1. Check for poor connections at the PCM.
  2. If a problem is found, repair as necessary. Refer to Wiring Repairs in Wiring Systems.

Did you perform a repair?

Go to Step 6

Go to Step 6

7
  1. Inspect the MIL control circuit for a short to ground.
  2. Repair as necessary. Refer to Wiring Repairs in Wiring Systems.

Did you complete the repair?

Go to Step 6

--

8
  1. Check the MIL circuit for the following conditions:
    2. • A malfunctioning bulb
    • A control circuit either open or shorted to B+
  2. Repair the conditions as needed. Refer to Wiring Repairs in Wiring Systems.

Did you complete the repair?

Go to Step 6

--

9

Replace the PCM. Refer to Powertrain Control Module Replacement .

Did you complete the replacement?

Go to Step 10

--

10
  1. Use the scan tool in order to clear the DTCs.
  2. Start the engine.
  3. Allow the engine to idle until the engine reaches the normal operating temperature.
  4. Select DTC and the Specific DTC function.
  5. Enter the DTC number that was set.
  6. Operate the vehicle within the Conditions for Setting this DTC until the scan tool indicates the diagnostic Ran.

Does the scan tool indicate the diagnostic passed?

Go to Step 11

Go to Step 2

11

Does the scan tool display any additional undiagnosed DTCs?

Go to Diagnostic Trouble Code (DTC) List

System OK