P071A

DTC P071A

Diagnostic Instructions

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

DTC Descriptor
DTC P071A
- Transmission Tow Mode Switch Circuit

Circuit/System Description

Tow/haul mode enables the operator to achieve enhanced shift performance when towing or hauling a load. When tow/haul mode is selected, the tow/haul switch input signal to the body control module (BCM) is momentarily toggled to zero volts. This signals the transmission control module (TCM) to extend the length of time between upshifts and increase transmission line pressure. Cycling the tow/haul switch again disables tow/haul mode and returns the transmission to a normal shift pattern.

Conditions for Running the DTC

* DTC P1762 is not set.
* The engine speed is greater than 500 RPM for 5 seconds.
* The system voltage is 9-18 volts.

Conditions for Setting the DTC

The Tow Mode switch remains closed, low voltage, for 10 minutes.

Action Taken When the DTC Sets

* DTC P071A is a Type C DTC.
* Tow Mode is disabled.

Conditions for Clearing the DTC

DTC P071A is a Type C DTC.

Diagnostic Aids

If the electrical circuit tests are OK and the shift pattern is not occurring, there may be a mechanical/hydraulic condition that prevents operation. Refer to Symptoms - Automatic Transmission (Symptoms - Automatic Transmission).

Reference Information
Schematic Reference

Automatic Transmission Controls Schematics (6T30/6T40/6T45 - Automatic Transmission)
Connector End View Reference

Component Connector End Views (Connector End Views By Name)
Description and Operation

Transmission General Description (Transmission General Description)
Electrical Information Reference

* Circuit Testing (Circuit Testing)
* Connector Repairs (Connector Repairs)
* Testing for Intermittent Conditions and Poor Connections (Testing for Intermittent Conditions and Poor Connections)
* Wiring Repairs (Wiring Repairs)

DTC Type Reference

Powertrain Diagnostic Trouble Code (DTC) Type Definitions (Powertrain Diagnostic Trouble Code (DTC) Type Definitions)
Scan Tool Reference

Control Module References (Programming and Relearning) for scan tool information
Special Tools

EL 35616 - GM-Approved Terminal Test Kit

For equivalent regional tools, refer to Special Tools (6T30/6T40/6T45 - Automatic Transmission).

Circuit/System Verification

Turn ON the ignition, with the engine OFF. Cycle the tow/haul switch while observing the Tow/Haul Mode parameter on the scan tool.

Circuit/System Testing

1. Ignition OFF, remove connector from the tow/haul switch.
2. Ignition ON, install a fused jumper wire between the tow/haul switch signal and ground circuits of the tow/haul switch connector. Use a scan tool to verify that the Tow/Haul Mode parameter status changes.

• If the scan tool Tow/Haul Mode indicates a status change, replace the tow/haul switch.

3. Using the EL 35616 - kit, install a fused jumper wire from the tow/haul switch signal circuit of the tow/haul switch connector to ground. Use a scan tool to verify that the Tow/Haul Mode parameter status changes.

• If Tow/Haul Mode status changes, test the tow haul switch ground circuit for an open/high resistance or short to ground.

• If the Tow/Haul Mode status did not change, test the tow haul mode signal circuit for an open/high resistance or a short to ground. If the circuit tests normal, replace the BCM.

Repair Instructions

Perform the Diagnostic Repair Verification (Verification Tests) after completing the repair.