What it means
Transmission Mode Switch B Circuit: an electrical fault is present in the circuit.
At a glance
- Usually lights the check-engine light
How it works
Transmission mode switch B provides the control module with a signal about a transmission mode or selector state. P071D sets when the module detects an electrical fault in switch B's circuit, which can result from the switch, its adjustment, or its wiring and connections.
Common causes
Most likely first.
- Faulty transmission mode switch B
- Open or short in switch wiring
- Poor connector contact or corrosion
- Misadjusted or damaged switch
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 B to see which selector position or operating condition coincides with the circuit fault.
- With the vehicle safely parked, compare the selector position with the switch B state shown by the scan tool and note any positions that are not recognized.
- Inspect switch B and its linkage or mounting for damage or misalignment, then check its connector and harness for corrosion, loose pins, or chafing.
- Move the selector through its positions while watching switch B live data to see whether the reported state changes consistently.
- Use the wiring diagram to check switch B circuit continuity and shorts, and test switch operation and adjustment against the manufacturer's specification.
Common fixes
- Clean and secure corroded or loose switch B connector terminals.
- Repair an open or shorted section of switch B wiring.
- Correct switch B adjustment or repair damaged mounting or linkage.
- Replace mode switch B if it fails its circuit or position tests.
Can I keep driving?
If shifts are harsh, delayed, or missing, limit driving because the transmission may not respond correctly to the selected mode. Stop if it slips or cannot shift predictably.
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.