Faulty transmission mode switch, with an internal short to earth or a stuck contact; Wiring in the switch circuit chafed or shorted to earth; Corroded, damaged or contaminated connector or terminals at the switch or control module; Liquid ingress in the switch, for example from a spilled drink in the centre console; Faulty transmission control module or the module that reads the switch, with an internal circuit fault; Fault in the shared reference or earth supply to the switch.

P071B

Transmission Mode Switch A Circuit Low.

Generic
Powertrain
Generic code
Low severity

How this code is built

P: SystemPowertrain
0: Defined bySAE, the same on every car
7: AreaTransmission
1B: FaultThe specific fault in that area

What it means

P071B is a generic powertrain code meaning the transmission control module has detected a low voltage or signal in the Transmission Mode Switch A circuit. The mode switch is the driver-operated control that selects modes such as sport, economy, snow or tow/haul. The module saw a circuit voltage below the expected range, which points to a short to earth, a fault in the switch itself or a wiring problem.

Symptoms

Warning light (MIL) on, possibly with a transmission or gearbox warning message
Selected drive mode does not change or does not match the switch position
Transmission stuck in a default mode, with unusual shift points or harsh or delayed shifts
Sport, snow or tow/haul mode unavailable
Poor fuel economy if the gearbox defaults to a different shift map

Possible causes

Faulty transmission mode switch, with an internal short to earth or a stuck contactlikely
Wiring in the switch circuit chafed or shorted to earthlikely
Corroded, damaged or contaminated connector or terminals at the switch or control modulepossible
Liquid ingress in the switch, for example from a spilled drink in the centre consolepossible
Faulty transmission control module or the module that reads the switch, with an internal circuit faultrare
Fault in the shared reference or earth supply to the switchrare

How to diagnose

Read all stored and pending codes and the freeze frame data, and note any other transmission, network or switch-related codes
Operate the mode switch through every position and watch the live data for the switch status, if the vehicle supports it
Visually inspect the switch, its surroundings and the connector for moisture, residue, damage or bent pins
Check the wiring harness between the switch and the control module for chafing, pinching or signs of a short to earth
Back-probe the circuit with a multimeter and compare the voltage with the manufacturer's specification at each switch position
Check the continuity of the circuit and earth, and the resistance of the switch against the manufacturer's values
Wiggle-test the harness and connector with the circuit live and watch for the signal dropping low
Only after the wiring and switch are proven good, check the module's power, earth and any published updates for your model

How to fix

Replace the transmission mode switch if it tests out of specification
Repair or replace chafed or shorted wiring, and secure the harness away from sharp edges
Clean or replace corroded or damaged connector terminals and refit the connector securely
Dry out and clean the switch area if there is moisture, and replace the switch if it is contaminated
Repair or replace the control module only if the fault is confirmed in the module and the wiring and switch are sound, and have it programmed as the manufacturer requires

Can I keep driving?

Low severity

The car can usually be driven, as the gearbox normally falls back to a default mode, but expect altered shifting and fewer drive modes. Book it in soon, and stop if shifting becomes harsh, erratic or the gearbox enters limp mode.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.