Error Code P071D is defined as Transmission Mode Switch B Circuit. Error Code P071D is a generic trouble code, meaning it applies to all vehicles equipped with the OBD-II system, especially those made since 1996 up to present. This includes vehicle models from but not limited to Buick, Chevrolet, Dodge, Ford, GMC, etc. Specifications on the definition, troubleshooting, and repairs, of course, vary from make, model, and powertrain configuration.

The TCM (transmission control module) monitors all the sensors and switches involved in the transmission’s operation. Vehicles with automatic transmission nowadays come with all the comfortable features.

For example, at times, the TCM monitors and controls the cruise control (among other modules). The tow/haul mode, for example, is enabled by the operator to modify the ratios and shift patterns, accommodating varying loads and towing demands. The process of this switch is imperative to the functionality of the tow/haul function among other systems. This can vary significantly from one manufacturer to another, so when diagnosing this code, make sure the switch applies to your current faulty and specific vehicle make and model.

The “B” in the description can have multiple different definitions. So make sure to get the right service information before performing any troubleshooting in your vehicle.

Error Code P071D is activated when the TCM detects a general fault in the “B” transmission mode switch or the circuit.

The PCM (powertrain control module, also known as ECM or engine control module in other vehicle makes) lights up the Check Engine light with this code. Generally, the tow/haul switch is located on or near the shift lever. For stalk-style shifter, it may be a button at the end of the lever. For console-style shifters, it may be located on the dash. Again, this varies between vehicles, so make sure to check with the service manual.

Other related error codes include:

Common Symptoms

  • A faulty or inefficient shifting pattern
  • Erratic or abnormal switch operation
  • Inoperative mode switch (i.e. tow/haul mode switch, sport mode switch, etc.)
  • Torque unable to downshift when needed
  • Low power under heavy load/towing

Possible Causes

  • Damaged or defective mode switch
  • Defective shift lever
  • A problem in the TCM
  • Wiring problem (frayed, shorted to ground, shorted to power, open, etc.)
  • Corrosion in connectors, grounds, pins, etc. causing high resistance
  • Fuse/box problem

How to Check

Basic Diagnosis Step 1

As with most codes, a scanner with monitoring capabilities (data stream) is needed to monitor the values and operation of the particular mode switch when diagnosing this code. Toggle the switch on and off to see if the scanner recognizes the inputs. There may be some lag, so a delay is always a good idea when monitoring the switches.

Also, if you see mode switch inoperative based on your scanner, you may be able to jump some pins at the connector of the mode switch to rule out the circuit. If the circuit is ruled out this way, and the switch remained inoperative, you can now test the switch itself.

Again, these are general guidelines. It’s best to refer to your manual for the specifications and exact diagnosis procedures.

Basic Diagnosis Step 2

If possible, test the switch. Many times, their job is to simply the right module(s) (i.e. TCM, BCM (body control module) PCM, etc.) your load/towing needs so that it can use modified shift patterns. With that said, most of these are in on/off style. Meaning, a simple continuity test using a DVOM (digital volt-ohmmeter scanner) can determine the functionality of the sensor. These sensors are sometimes incorporated into the shift lever, so make sure to research thoroughly which connectors or points you need to check with your multimeter.


As with any transmission fault, make sure there are adequate levels and quality of fluid in the transmission.

How to Fix

Depending on the diagnosis, common repairs for this code include:

  • Replacement of mode switch
  • Cleaning or replacement of corroded connectors, pins, ground, etc.
  • Repair or replacement of wiring problems (shorted, open, etc.)
  • Replacement of damaged shift lever
  • TCM repair

The severity of Error Code P071D refers to what mode switch the fault is referring to. In the case of tow/haul switches, it’s usually a minor problem. With this, however, you must avoid heavy towing or loads, as it can put your transmission and its accompanying components to unnecessary stress.