P0866

TCM Communication Circuit High

Q: 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.

Q: Can I just clear P0866?

It returns until the wiring, connector, or module fault behind the communication error is repaired.

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.

Real-world scan data

Seen in

scans in the past year

Found in

<0.1%

of scans

<0.1%

of scans that found faults

Figures reflect only diagnostic scans recorded through our own apps and tools. Cars are typically scanned because a symptom or warning light prompted it, so these counts describe our scan records — not the incidence of faults in any model, and not the quality, reliability, or safety of any vehicle. No comparison between manufacturers or models is implied. All trademarks belong to their respective owners; no affiliation or endorsement is implied.

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.

Read & clear P0866 yourself

Scan your car, confirm the code, and clear it once it's fixed — with OBD DashX on your phone.

▶ Get it on Google Play

✓ Verified name — SAE J2012 / ISO 15031-6Guidance reviewed by OBD DashX