GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Cadillac 🢒 2008 🢒 Escalade ESV 🢒 DTC 🢒 DTC U0074

Diagnostic Instructions
    • Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.
    • Review Strategy Based Diagnosis for an overview of the diagnostic approach.
    •  Diagnostic Procedure Instructions provides an overview of each diagnostic category.

DTC Descriptor

DTC U0074 00: Control Module Communication Bus B Off

Diagnostic Fault Information

Circuit

Short to Ground

Open/High Resistance

Short to Voltage

Signal Performance

PTLAN +

U0074*

U189A* **

U0074*

--

PTLAN -

U0074*

U189A* **

U0074*

--

* Other modules/DTCs will set with this fault.

** An open between the on either PTLAN circuit may also cause a DTC U0074.

Circuit/System Description

The PTLAN serial data circuits are serial data buses used to communicate information between the control modules. The PTLAN serial data circuits also connect directly to the data link connector (DLC) for programming purposes. If this serial data system is inoperative the modules will still communicate with the scan tool due to other serial data systems but not with each other. Communication with the scan tool for the modules on the PTLAN bus is through the serial data gateway module (SDGM) on the HSGMLAN serial data system.

Conditions for Running the DTCs
    • Supply voltage to the modules are in the normal operating range.
    • The ignition switch in any position other than START.

Conditions for Setting the DTC

The module setting the DTC has attempted to establish communications on the PTLAN serial data circuits more than 3 times.

Action Taken When the DTCs Sets
    • The module suspends all message transmission.
    • The MIL and battery indicators will illuminate, along with several DIC messages.
    • The vehicle may continue to run for several minutes, then will stall and not start.

Conditions for Clearing the DTC
    • A current DTC clears when the malfunction is no longer present.
    • A history DTC clears when the module ignition cycle counter reaches the reset threshold, without a repeat of the malfunction.

Diagnostic Aids
    • When testing circuits to the SDGM access can be accomplished by removing the instrument panel center trim panel. Note that this procedure does not allow removal of the SDGM, only access to the connector. Refer to Instrument Panel Center Trim Panel Replacement .
    • Circuits to the HPCM are diagnosed only to the PIM connector because the HPCM is replaced as part of the PIM.
    • Different DTCs may set in different modules along with this DTC depending on where the fault occurs and the vehicle operating conditions when the fault occurs.

Reference Information

Schematic Reference

    •  Data Communication Schematics
    •  Control Module References

Connector End View Reference

Component Connector End Views

Description and Operation

Data Link Communications Description and Operation

Electrical Information Reference

    •  Circuit Testing
    •  Connector Repairs
    •  Testing for Intermittent Conditions and Poor Connections
    •  Wiring Repairs

Circuit/System Testing

Initial Test

  1. Ignition ON, verify 4-6 volts between the PTLAN circuits of the DLC connector terminals 3 and 11.
    2. ⇒If not within the specified range, refer to Active U0074 Test.
  2. Verify that DTC U18xx is not set in any module listed.
    3. • ECM
    • HPCM
    • SDGM
    ⇒If DTC U18xx is set in any of the above modules, refer to Diagnostic Trouble Code (DTC) List - Vehicle .

Active U0074 Test

  1. With the ignition OFF, disconnect the module/component harness connectors listed below one at a time. After each disconnect and with the ignition ON, test for 4-6 volts between PTLAN circuits of DLC connector terminals 3 and 11 until the voltage is within specified range.
    2. 1.1. Data link resistor 2 - X1
    1.2. Yaw rate and lateral acceleration sensor - X1
    1.3. Engine control module (ECM) - X3
    1.4. Hybrid powertrain control module (HPCM) - X1
    1.5. Accessory DC power control module (APM) - X1
    1.6. Electronic brake control module (EBCM) - X1
    1.7. Serial data gateway module (SDGM) - X1
    ⇒If all module/components are disconnected and not within the specified range, test all 4 PTLAN circuits of SDGM connector X1 for a short between them, short to voltage, or a short to ground. If the circuits test normal, replace the SDGM.

    Important: If either the data link resistor or the ECM is the module/component disconnected before voltage at the DLC terminals is within the specified range no further testing is necessary, replace the last module/component disconnected.

  2. Test for infinite resistance between both PTLAN circuits of the last module disconnected that lead to the previously disconnected module/component.
    3. ⇒If less than the specified value, test the circuits for a short together.
  3. Test for infinite resistance between ground and both PTLAN circuits of the last module disconnected that lead to the previously disconnected module/component.
    4. ⇒If less than the specified value, test the circuit for a short to ground.
  4. Test for less than 1 volt between ground and both PTLAN circuits of the last module disconnected that lead to the previously disconnected module/component.
    5. ⇒If greater than the specified range, test the circuit for a short to voltage.
  5. If the circuits test normal, replace the last module disconnected.

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

    •  GMLAN Wiring Repairs
    •  ABS Description and Operation for the ABS - Hybrid
    •  Vehicle Yaw Sensor with Vehicle Lateral Accelerometer Replacement for the ABS - Non-Hybrid Less than 3900 kg (8600 lb) GVW
    •  Vehicle Yaw Sensor with Vehicle Lateral Accelerometer Replacement for the ABS - Non-Hybrid Greater than/Equal to 3900 kg (8600 lb) GVW
    •  Control Module References for module replacement, setup, and programming