GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2004 🢒 Canyon Pickup - 4WD 🢒 DTC 🢒 DTC B1372, B1373, or B1374

Circuit Description

The body control module (BCM) monitors the ignition inputs Ignition Off/Run/Crank, Ignition 1 Run/Crank and the accessory signals supplied from the ignition switch. The BCM uses the sequence that the signals appear and their voltage levels to determine the power mode called for by the vehicle operator using the ignition switch. Battery voltage is supplied to the ignition switch by the IGN TRANSD fuse. The ignition switch uses this voltage for Run/Crank signal discrimination. The Ignition 1 Run/Crank signal also energizes the Run/Crank relay. If the BCM detects that the Ignition 1 Run/Crank signal circuit is open or shorted, one of the following DTCs will set:

DTC Descriptors
    • DTC B1372 Device Ignition 1 (ON and START) Circuit Low
    • DTC B1373 Device Ignition 1 (ON and START) Circuit High

Conditions for Running the DTC
    • The BCM detects a change in the ignition switch circuit states.
    • Battery voltage is 9.0-18.5 volts.

Conditions for Setting the DTC

DTC B1372

The BCM detects that the Ignition 1 Run/Crank signal is open or shorted to ground for at least 10 seconds.

DTC B1373

The BCM detects that the Ignition 1 Run/Crank signal is shorted to voltage for at least 10 seconds.

Action Taken When the DTC Sets
    • The BCM stores DTC B1372 or B1373 in memory.
    • The BCM operates in a fail-safe power mode dependent on the last valid power mode detected and the state of the engine run flag data on the class 2 serial data communications circuits.
    • The other modules on the vehicle operate in a fail safe power mode dependent on the last valid power mode transmitted by the BCM and the state of the engine run flag on the class 2 serial data communications circuits.

Conditions for Clearing the DTC
    • A current DTC B1372 or B1373 is moved to history when the ignition is cycled.
    • A history DTC B1372 or B1373 clears when the ignition has been cycled 100 times, without a repeat of the malfunction.

Step

Action

Yes

No

Schematic Reference: Body Control System Schematics

Connector End View Reference: Body Control System Connector End Views and Master Electrical Component List in Wiring Systems

1

Did you perform the Diagnostic System Check for the system exhibiting the symptom?

Go to Step 2

Go to Control Module References for the applicable Diagnostic System Check

2
  1. Install a scan tool.
  2. Observe the Ignition 1 Run/Crank parameter with the scan tool.
  3. Turn the ignition ON, with the engine OFF.
  4. Turn the ignition OFF.

Does the scan tool display Ignition 1 Run/Crank Active and Inactive?

Go to Testing for Intermittent Conditions and Poor Connections in Wiring Systems

Go to Step 3

3

  1. Place the ignition switch in the OFF position, leaving the key in the ignition.
  2. Disconnect the body control module (BCM) harness connector J2.
  3. Probe the Ignition 1 Run/Crank circuit with a test lamp connected to a good ground.
  4. Probe the Ignition 1 Run/Crank circuit with a test lamp connected to B+.

Does the test lamp illuminate for either test?

Go to Step 7

Go to Step 4

4

  1. Turn the ignition switch to the RUN position.
  2. Probe the Ignition 1 Run/Crank circuit at the BCM J2 connector with a test lamp connected to a good ground.
  3. Turn the ignition switch to CRANK.

Does the test lamp illuminate in both RUN and CRANK?

Go to Step 5

Go to Step 8

5

  1. Turn the ignition OFF.
  2. Disconnect the BCM harness connector J1.
  3. Probe the Ignition 1 Run/Crank circuit in the J1 connector with a test lamp connected to a good ground.
  4. Probe the Ignition 1 Run/Crank circuit with a test lamp connected to B+.

Does the test lamp illuminate for either test?

Go to Step 6

Go to Step 10

6
  1. Remove the Run/Crank relay. Refer to Relay Replacement in Wiring Systems.
  2. Test the Ignition 1 Run/Crank circuit between the BCM and the Run/Crank relay for a short to ground or short to voltage. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 14

Go to Step 11

7
  1. Disconnect the ignition switch harness connector.
  2. Test the Ignition 1 Run/Crank circuit and the ignition switch voltage supply circuit for a short to ground or short to voltage. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 14

Go to Step 12

8

Test the Ignition 1 Run/Crank circuit for an open or a high resistance. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 14

Go to Step 9

9

Inspect for poor connections at the harness connector of the ignition switch. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 14

Go to Step 12

10

Inspect for poor connections at the BCM. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 14

Go to Step 13

11

Replace the Run/Crank relay. Refer to Relay Replacement in Wiring Systems.

Did you complete the replacement?

Go to Step 14

--

12

Replace the ignition switch. Refer to Ignition and Start Switch Replacement in Steering Wheel and Column.

Did you complete the replacement?

Go to Step 14

--

13

Important: Perform the programming/RPO Configuration for the replacement BCM. Refer to Body Control Module (BCM) Programming/RPO Configuration .

Replace the BCM. Refer to Body Control Module Replacement .

Did you complete the replacement?

Go to Step 14

--

14
  1. Reconnect all disconnected connectors.
  2. Turn the ignition ON, with the engine OFF.
  3. Clear the DTCs with the scan tool.
  4. Operate the vehicle within the conditions for running the DTCs as specified in the supporting text.

Does DTC B1372 or B1373 reset?

Go to Step 2

System OK