P0866
TCM Communication Circuit High
Transmission
Generic OBD-II
Medium
What it means
P0866 means a higher-than-expected voltage on the communication circuit between the transmission control module (TCM) and the rest of the vehicle's network. The engine still starts, but the TCM may lose reliable data, so the transmission can shift erratically or drop into limp mode. It usually points to wiring, a corroded connector, or a power/ground problem rather than the module itself.
Medium
Can I keep driving? The engine starts, but a communication fault can put the transmission in limp mode or stick it in one gear — drive cautiously and have it checked soon.
Common symptoms
What you'll notice
- Check-engine light on
- Delayed, erratic, or missing shifts
- Transmission stuck in one gear (limp mode)
- Other transmission warning messages
Common causes
Most likely first
- Wiring fault or short causing high voltage on the TCM communication circuitLikely
- Loose or corroded connector at the TCMCommon
- Poor ground or power supply to the TCMCommon
- Faulty transmission control module (TCM)Less often
- Fault in the ECM or another module on the same networkLess often
How to diagnose & fix
Work through it in order
- Inspect the TCM connector and communication wiring for damage, corrosion, or a short-to-voltage.
- Verify the TCM's power and ground circuits are clean and within spec.
- Check the network for other modules reporting communication faults to narrow down the source.
- Test the communication circuit voltage against spec.
- Replace the TCM only after wiring, power, ground, and network causes are ruled out. Clear and re-test.
FAQ
Quick answers
Is it safe to drive with P0866?+
Short distances at reduced speed are usually okay, but limp mode or an unexpected gear can be unsafe in traffic, so get it diagnosed soon.
Can I just clear P0866?+
It returns until the wiring, connector, or module fault behind the communication error is repaired.
Related codes
Often seen with P0866
Read & clear P0866 yourself
Scan your car, confirm the code, and clear it once it's fixed — with OBD DashX on your phone.
✓ Verified name — SAE J2012 / ISO 15031-6Guidance reviewed by OBD DashX