Technical Description of OBD-II Trouble Code
Circuit B of the Transmission Mode Switch
What does that signify?
This is a standard diagnostic trouble code (DTC) for powertrain and is commonly found in OBD-II vehicles. This can include vehicles from various manufacturers such as GMC, Chevrolet, Ford, Buick, Dodge, etc. While it is a generic code, the specific repair steps may differ based on the year, make, model, and powertrain configuration.
The Transmission Control Module (TCM) monitors all the sensors and switches involved in the operation of the transmission. Nowadays, automatic transmissions (also known as A/T) offer more convenience features than ever before.
For example, in some cases, the TCM controls and monitors the cruise control along with other modules. In this article, I will focus on the tow/haul mode, which allows the operator to adjust the ratios and shift patterns to accommodate different loads and towing requirements. The proper functioning of this switch is crucial for the tow/haul function and other related systems. However, the specific mode switch can vary significantly between manufacturers, so it is important to identify the correct one for your particular fault and make/model.
The letter “B” in this code can have multiple definitions or distinguishing factors. These definitions often vary, so it is essential to obtain the appropriate service information before conducting any extensive troubleshooting. This is not only important but also necessary to accurately diagnose uncommon or obscure faults. Consider this article as a learning tool due to its general nature.
The ECM triggers the MIL (Malfunction Indicator Lamp) and displays the codes P071D and/or related codes (P071E, P071F) when it detects a malfunction in the mode switch. In most cases, tow/haul switches are located on or near the shift lever. In stalk-style shifters, it may be a button at the end of the lever, while in console-style shifters, it may be located on the dashboard. The location can vary significantly between vehicles, so consult your service manual for precise information.
The P071D code for Circuit B of the Transmission Mode Switch is activated when the ECM (Engine Control Module) and/or TCM detects a general fault in the “B” transmission mode switch or its circuit.
An example of a stalk-mounted transmission tow/haul mode switch:
What is the severity of this DTC?
The severity of this DTC depends on the specific mode switch associated with the vehicle’s fault. In the case of tow/haul switches, the severity is considered low. However, it is advisable to avoid heavy loads and/or towing to prevent unnecessary stress on the transmission and its components. Use common sense in these situations.
What are some of the symptoms of the code?
The symptoms of a P071D trouble code may include:
What are some of the common causes of the code?
The causes of the P071D code may include:
What are some P071D troubleshooting steps?
Basic Step #1
The starting point for troubleshooting may vary depending on the tools and references available to you. If your scanner has monitoring capabilities (DATA STREAM), you can use it to monitor the values and operation of your specific mode switch. Toggle the switch on and off to see if your scanner detects the inputs. Keep in mind that there may be a slight delay, so wait a few seconds when monitoring switches.
In addition, if your scanner indicates that the mode switch is inoperative, you can try jumping some pins at the switch’s connector to eliminate the circuit as the cause. If the circuit is ruled out using this method and the switch is still not functioning, proceed to test the switch itself. These are general guidelines, and with a capable scan tool, troubleshooting can be relatively easy if you know what to look for. Refer to the service manual for specifications and procedures.
Basic Step #2
If possible, test the switch itself. In most cases, these switches simply signal the appropriate module(s) (e.g., TCM, BCM, ECM) about your towing/load requirements so that modified shift patterns can be implemented. The majority of these switches are on/off style. Therefore, a simple continuity test using an ohm meter can determine the functionality of the sensor. Keep in mind that these sensors are sometimes integrated into the shift lever, so research which connectors/pins you need to monitor with your multimeter.
NOTE: As with any transmission fault, always ensure that fluid levels and quality are adequate and well-maintained.
