What it means
Same as P071A with the signal shorted to ground. Wiring faults of this kind typically come from chafing, water ingress at connector seals, or pin push-back — a wiggle test on the harness while the code is active often confirms the location.
At a glance
- Usually lights the check-engine light
How it works
Transmission mode switch A reports a selected transmission mode to the control module. The module monitors switch A's signal voltage and sets P071B when that signal stays below its expected range, consistent with a signal pulled toward ground.
Common causes
Most likely first.
- Signal shorted to ground
- Connector pins corroded - high contact resistance
- Harness chafing through insulation
Symptoms
- Check engine or transmission warning light on
- Harsh, delayed, or missing shifts
- Possible limp mode (locked in one gear)
- Slipping or shuddering during gear changes
How to diagnose it
- Read freeze-frame data and live data for mode switch A to see when its reported state or signal drops low and whether the fault is present now.
- With the vehicle safely parked, compare the indicated mode with the selector position and check whether the transmission warning light or shift symptoms change as the selector is moved.
- Inspect switch A's connector and harness for water, corrosion, pushed-back pins, or insulation rubbed through to ground.
- Watch switch A's live data while gently moving the accessible harness to see whether the low reading appears or changes at a particular spot.
- Use the wiring diagram to test switch A's signal for a short to ground and check connector contact and switch operation against the manufacturer's specification.
Common fixes
- Clean and secure corroded or loose switch A connector terminals.
- Repair chafed signal wiring that is contacting ground.
- Repair damaged connector seals or terminals that allow moisture or poor contact.
- Replace a faulty mode switch A if its signal remains low with the wiring verified.
Can I keep driving?
Avoid relying on the transmission's mode selection if shifts are harsh, delayed, or missing; the transmission may enter a protective gear strategy. If it slips or cannot shift predictably, stop driving and arrange diagnosis.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.