AutoCodes – Generic – P0866

P0866 Code - TCM Communication Circuit High

Quick Answer

The Engine Control Module (ECM) monitors the Transmission Control Module (TCM). Common causes include Faulty Transmission Control Module (TCM), Transmission Control Module (TCM) harness is open or shorted, Transmission Control Module (TCM) circuit poor electrical connection. Severity is High - diagnose this code as soon as possible. Driving is not recommended if the warning light is flashing, the engine runs poorly, or safety-related symptoms are present.

What it means: P0866 indicates TCM Communication Circuit High.
Common causes: Common causes include Faulty Transmission Control Module (TCM), Transmission Control Module (TCM) harness is open or shorted, Transmission Control Module (TCM) circuit poor electrical connection.
Severity: High - diagnose this code as soon as possible.
Can I drive?: Driving is not recommended if the warning light is flashing, the engine runs poorly, or safety-related symptoms are present.

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Code Popularity 10,071

How popular is this code, among other codes? Low

Repair Importance Level

Urgent (Diagnose now) (3 / 3)

Repair Difficulty Level

Advanced (Pro) (3 / 3)

What is this?

Table of contents Created and updated by William Orellana on May 11, 2019

Description
Possible Causes
How To Fix
Cost to Diagnose
FAQs
Comments
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Code P0866 Description

The Engine Control Module (ECM) monitors the Transmission Control Module (TCM). The ECM sets the OBDII when there is a problem communicating with the TCM.

OBDII Code P0866 - TCM Communication Circuit High - AutoCodes.com — P0866 Code - TCM Communication Circuit High

What does this mean?

What are the Possible Causes of the DTC P0866?

Faulty Transmission Control Module (TCM)
Transmission Control Module (TCM) harness is open or shorted
Transmission Control Module (TCM) circuit poor electrical connection
Faulty Engine Control Module (ECM)

How to Fix the DTC P0866?

Review the 'Possible Causes' above and visually examine the corresponding wiring harness and connectors. Check for damaged components and inspect connector pins for signs of being broken, bent, pushed out, or corroded.

What is the Cost to Diagnose the Code?

Labor: 1.0

To diagnose the P0866 code, it typically requires 1.0 hour of labor. Rates vary by location, vehicle, and shop. Many shops charge between $80–$150 per hour; dealerships and metro areas may be higher, independents may be lower.

Need more information about the P0866 code?

Need more information on how to fix the P0866 code? Get Access to Factory Service Manuals

Need Help with the Code?

Get the P0866 code diagnosed by a professional: Find a repair shop in your area

Information for Specific Makes

P0866 Dodge — TCM Communication Circuit High

About the author: William Orellana is a Master Mechanic and Mechanical Engineer with over 30 years of experience diagnosing and repairing vehicles. Learn more about his background.

Frequently Asked Questions about P0866 Code

1. What are the common symptoms of OBDII code P0866 (TCM Communication Circuit High)?

The common symptoms of P0866 include difficulty shifting gears, transmission slipping, illuminated Check Engine Light, and in some cases, the vehicle may go into limp mode.

2. What are the possible causes of OBDII code P0866 (TCM Communication Circuit High)?

The potential causes of code P0866 include faulty wiring or connectors in the transmission control module (TCM) circuit, a malfunctioning TCM, low transmission fluid levels, or a defective transmission control module.

3. How can I diagnose OBDII code P0866?

Diagnosing code P0866 involves using an OBDII scanner to retrieve the trouble code, inspecting the wiring and connectors in the TCM circuit, checking the transmission fluid level and condition, and performing any necessary tests on the TCM.

4. Can I continue driving with OBDII code P0866 present?

It is not recommended to continue driving with OBDII code P0866 present, as it can lead to further damage to the transmission and other components. It is best to address the issue promptly to prevent more extensive repairs.

5. How can I repair OBDII code P0866?

Repairing OBDII code P0866 typically involves addressing the root cause of the issue, such as repairing or replacing faulty wiring or connectors, diagnosing and fixing any TCM malfunctions, ensuring proper transmission fluid levels, and clearing the trouble code.

6. Is it possible to reset OBDII code P0866 without fixing the underlying issue?

Resetting the trouble code without fixing the underlying issue will not solve the problem causing code P0866. It is essential to diagnose and repair the root cause to prevent the code from reoccurring.

7. Can a DIYer fix OBDII code P0866 at home?

Depending on the DIYer's skill level and access to tools, it may be possible to address some aspects of code P0866 at home, such as checking wiring connections or fluid levels. However, diagnosing and repairing TCM or other complex issues may require professional assistance.

8. How much does it cost to repair OBDII code P0866?

The cost of repairing OBDII code P0866 can vary depending on the underlying issue, parts needed, labor costs, and the shop's hourly rate. It's recommended to obtain multiple quotes from reputable repair shops for an accurate estimate.

9. Will OBDII code P0866 cause permanent damage to my vehicle?

If left unaddressed, OBDII code P0866 can potentially cause permanent damage to the transmission or other related components. Prompt diagnosis and repair are crucial to prevent further issues.

10. How long does it take to repair OBDII code P0866?

The repair time for OBDII code P0866 can vary based on the specific issue causing the code and the complexity of the repair. Some fixes may be relatively quick, while others may take more time for diagnosis and parts replacement.