P0866 — TCM Communication Circuit High

What Does P0866 Mean?

P0866 is a diagnostic trouble code indicating: TCM Communication Circuit High. This code relates to the transmission system. The vehicle's computer detected a condition outside normal operating parameters and stored this code.

Symptoms of P0866

⚠️ Check Engine Light (MIL) illuminated
⚠️ TCM Communication reading stuck at maximum value
⚠️ Related system operating erratically or in default mode
⚠️ Harsh or delayed shifting

Common Causes

🔍 Short to voltage in TCM Communication wiring harness
🔍 Corroded or water-damaged TCM Communication connector
🔍 Faulty TCM Communication outputting maximum voltage
🔍 Damaged wiring rubbing against metal components
🔍 ECM/PCM internal fault (rare)

Frequently Asked Questions

What does P0866 mean?

P0866 indicates: TCM Communication Circuit High. The vehicle's computer has detected that the TCM Communication is not operating within its expected parameters and stored this diagnostic trouble code.

Can I drive with code P0866?

Only short distances if absolutely necessary. P0866 affects a critical system and should be diagnosed within a few days to avoid further damage.

How much does it cost to fix P0866?

Repair costs for P0866 typically range from $20 to $500, depending on the root cause. Simple wiring or connector repairs are on the lower end, while component replacement costs more. A proper diagnosis ($80–$150 at most shops) will identify the exact cause before committing to expensive parts.

What To Do Next

Possible Fixes

🔧 Inspect and repair TCM Communication wiring for short to voltage
🔧 Clean or replace corroded TCM Communication connector
🔧 Replace faulty TCM Communication

1
Find your vehicle above
Click your make and model for real owner reports and common causes specific to your vehicle.
2
Check for recalls
Go to nhtsa.gov/recalls and enter your VIN. Free repairs if covered.
3
Get a professional diagnosis
A code alone doesn't identify the exact failed part. A diagnostic ($50–$150) pinpoints the root cause.

P0866: TCM Communication Circuit High