P0866 Fault Code

TCM Communication Circuit High

• What does that mean?
• Symptoms
• Causes
• Solutions

The P0866 fault code indicates an issue with the Transmission Control Module (TCM) communication circuit being too high. This can disrupt the communication between the TCM and other vital vehicle components, potentially leading to improper transmission function.

What are the symptoms of the P0866 code?

• Erratic shifting or inability to shift
• Warning lights illuminated on the dashboard (e.g., check engine light)
• Sluggish vehicle performance or poor acceleration
• Transmission slipping or unintended behavior
• Vehicle going into limp mode to protect the transmission

What causes the P0866 code?

• Faulty transmission control module
• Poor electrical connection or corroded wiring in the TCM circuit
• Short circuit or open circuit in the wiring harness
• Issues with the vehicle’s battery or charging system
• Failed transmission sensors that affect communication

How to fix P0866?

• Inspect and repair wiring and connectors in the TCM circuit
• Replace the transmission control module if faulty
• Test the vehicle’s battery and charging system for proper operation
• Clear any existing error codes and retest to see if the P0866 code returns
• If necessary, update or reprogram the TCM with the latest software versions