Technical Description of OBD-II Trouble Code
Circuit A of the Transmission Mode Switch
What does that signify?
This is a generic 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 vary depending on the year, make, model, and powertrain configuration of the vehicle.
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) have more advanced features than ever before.
For example, in some cases, the TCM may also monitor and control cruise control along with other modules. In this article, I will focus on the tow/haul mode, which is enabled by the driver to adjust the ratios and shift patterns for 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 details of this switch can vary significantly between manufacturers, so it is important to identify the correct mode switch for your specific fault and vehicle make and model.
The letter “A” in this code can have multiple definitions or distinguishing factors, which may vary in different situations. Therefore, it is essential to obtain the appropriate service information before conducting any troubleshooting steps. 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 illuminates the MIL (Malfunction Indicator Lamp) with the P071A code and/or related codes (P071B, P071C) when it detects a malfunction in the mode switch. In most cases, tow/haul switches are located on or near the shift lever. In a stalk-style shifter, 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 of the switch can vary significantly between vehicles, so consult your service manual for specific details.
The P071A code for the Transmission Mode Switch A Circuit is triggered when the ECM (Engine Control Module) and/or the TCM detects a general fault in the “A” transmission mode switch or its circuit.
Here is 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 that is causing the fault in your vehicle. In the case of tow/haul switches, I would classify the severity as low. However, it is advisable to avoid heavy loads and towing to prevent unnecessary stress on the transmission and its components. Use common sense in this regard.
What are some of the symptoms of the code?
The symptoms of a P071A trouble code may include:
What are some of the common causes of the code?
The causes of the P071A code may include:
What are some P071A 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 test the circuit. 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 moderately 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 are used to 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, you can use an ohm meter to perform a simple continuity test and determine the functionality of the sensor. Keep in mind that these sensors are sometimes integrated into the shift lever, so make sure to 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.
