01 ABS & Transmission: The Unexpected Connection You Might Not… • 02 What Depth of Water Can I Drive Through With My Car? • 03 Overheating, Misfires, and More: Common Engine Problems Cau… • 04 Before You Open A Car Dealership, Here Are Things Nobody Te… • 05 This Is Why Your Car’s Oil Filter Is More Important Than Yo… • 06 What’s Dripping Under My Car? Identify Fluid Leaks By Colour • 07 How Tire Pressure Affects the Wear and Tear of Your Tires • 08 P0013 B Camshaft Position - Actuator Circuit (Bank 1) • 09 Car Warranty Secrets: What Dealers Don't Want You to Know • 10 How to Rebuild a Car Battery (Is It Worth the Effort?) •

DTC codes / P07xx · Transmission / P071C

P071CTransmission Mode Switch A Circuit High

P071C indicates that the transmission control module (TCM) has detected an abnormally high voltage on the signal circuit for Transmission Mode Switch A, which is typically a driver-selectable switch for sport, economy, or winter driving modes. The TCM expects a specific voltage range on this circuit and sets the code when the voltage exceeds the upper threshold, indicating an open circuit, short to power, or a faulty switch.

SystemPowertrain
Code typeGeneric (all makes)
SeverityMedium
Fix difficultyIntermediate
AreaTransmission

Symptoms you may notice

  • The transmission mode indicator light may flash or remain off, and the selected mode may not engage.
  • The vehicle may default to a single driving mode, often normal or economy, regardless of switch position.
  • Some drivers notice no symptoms besides the warning light, especially if the default mode is acceptable.
  • Harsh or delayed shifts may occur as the TCM limits mode-dependent shift strategies.
  • The transmission may stay in a single gear or enter limp mode in severe cases.
  • Other dashboard warnings, such as stability control or check engine light, may illuminate if the mode switch shares data with other systems.

Common causes, most likely first

  1. A faulty transmission mode switch that internally shorts to voltage or has excessive resistance.
  2. An open or high-resistance connection in the signal circuit, often due to a broken wire or corroded connector.
  3. A short to voltage on the signal wire, which can be caused by chafed insulation contacting a power wire.
  4. A poor ground connection at the switch or TCM, leading to reference voltage errors.
  5. A malfunctioning TCM that misinterprets the signal voltage, though this is less common.
  6. Aftermarket accessories or improper wiring repairs that introduce voltage into the circuit.

How to diagnose and fix it

  1. Use a scan tool to verify the code and check for any related codes in the TCM and other modules.
  2. Inspect the transmission mode switch connector and wiring for corrosion, damage, or loose pins.
  3. Measure the signal voltage at the switch with the key on and compare it to the manufacturer's specification.
  4. Disconnect the switch and check for an open or short in the harness between the switch and TCM using a multimeter.
  5. Test the switch operation with an ohmmeter or scan tool data to confirm it changes state correctly.
  6. If the switch fails testing, replace it and clear the code to verify the repair.
  7. If the wiring and switch are good, check TCM grounds and power feeds, and consider TCM software updates or replacement as a last resort.

Mode switch faults often mimic wiring issues

  • A common oversight is replacing the TCM when the actual fault is a chafed signal wire shorting to a nearby power wire in the console or under the dash.
  • The mode switch may share a ground with other console components, so a bad ground can set P071C along with unrelated codes.
  • Some vehicles use a multiplexed signal where the switch sends a variable voltage; a high voltage can indicate a stuck switch rather than an open circuit.
  • Aftermarket remote starters or alarm systems tapped into the console wiring frequently introduce voltage spikes that trigger this code.
  • The code can set intermittently if the switch is only faulty in certain positions, so testing should include wiggling the harness and operating the switch through all modes.

Can I keep driving with P071C?

The vehicle is generally safe to drive because the transmission defaults to a limp or normal mode, but avoid towing or aggressive driving until the fault is repaired. If the transmission enters limp mode or shifts harshly, seek service promptly to prevent further damage.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

Running on fumes Free guides aren't free to make. Top up the tank. Any amount, one time. Fuel the Garage