OBD-II Trouble Code Technical Description
The circuit for Transmission Mode Switch A is experiencing a low electrical condition.
What does that mean?
This is a generic powertrain diagnostic trouble code (DTC) that is commonly found in OBD-II vehicles, including those from GMC, Chevrolet, Ford, Buick, Dodge, and others. However, the specific repair steps may vary depending on the year, make, model, and powertrain configuration of the vehicle.
The Transmission Control Module (TCM) is responsible for monitoring all the sensors and switches involved in the operation of the transmission. In modern automatic transmissions, there are various features and functions that are controlled by the TCM, such as cruise control and tow/haul mode. The tow/haul mode is used to adjust the transmission ratios and shift patterns to accommodate different loads and towing demands. The proper functioning of the mode switch is crucial for the tow/haul function and other related systems. It is important to identify the specific mode switch that is associated with the fault in order to address the issue correctly.
The letter “A” in this code can have different definitions or distinguishing factors, which may vary depending on the situation. It is essential to consult the appropriate service information to accurately troubleshoot uncommon or obscure faults. This article serves as a general guide and a learning tool.
The ECM (Engine Control Module) will activate the MIL (Malfunction Indicator Lamp) and display the P071B code, along with related codes (P071A, P071C), when it detects a malfunction in the mode switch. The location of the tow/haul switch can vary depending on the vehicle, so it is advisable to refer to the service manual for its specific location.
The P071B code indicates a low electrical condition in the circuit of the “A” transmission mode switch, as detected by the ECM and/or TCM.
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 associated with the vehicle’s fault. In the case of the tow/haul switch, the severity is considered low. However, it is advisable to avoid heavy loads and towing to prevent unnecessary stress on the transmission and its components.
What are some of the symptoms of the code?
The symptoms of a P071B trouble code may include:
What are some of the common causes of the code?
The common causes of the P071B code may include:
What are some P071B troubleshooting steps?
Basic Step #1
The starting point for troubleshooting may vary depending on the available tools and references. If your scanner has monitoring capabilities (DATA STREAM), you can use it to monitor the values and operation of the mode switch. Toggle the switch on and off to see if the scanner recognizes the inputs. Keep in mind that there may be a slight delay in the response, so wait a few seconds when monitoring switches.
If the mode switch appears to be inoperative according to the scanner, you can try jumping some pins at the switch’s connector to test the circuit. If the circuit is ruled out and the switch is still not functioning, proceed to test the switch itself. These are general guidelines, and it is recommended to consult the service manual for specific specifications and procedures.
Basic Step #2
If possible, test the mode switch itself. In most cases, these switches are simple on/off style switches that signal the appropriate module(s) (e.g. TCM, BCM, ECM) about the towing/load requirements. You can use an ohm meter to perform a continuity test and determine the functionality of the switch. Keep in mind that some switches may be incorporated into the shift lever, so research the appropriate connectors/pins to monitor with a multimeter.
NOTE: Always ensure that the fluid levels and quality in the transmission are adequate and well-maintained when dealing with any transmission fault.
