AutoCodes – Generic – P1866

P1866 Code - Transmission Transfer Case System Concern Servicing Required

Quick Answer

The Transmission Control Module (TCM) monitors the Transmission Transfer Case System. Common causes include Faulty Transfer Case Shift Motor, Transfer Case Shift Motor harness is open or shorted, Transfer Case Shift Motor 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: P1866 indicates Transmission Transfer Case System Concern Servicing Required.
Common causes: Common causes include Faulty Transfer Case Shift Motor, Transfer Case Shift Motor harness is open or shorted, Transfer Case Shift Motor 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 19,548

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 Jan 31, 2020

Description
Possible Causes
How To Fix
Cost to Diagnose
Comments
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Code P1866 Description

The Transmission Control Module (TCM) monitors the Transmission Transfer Case System. The TCM sets the OBDII code when the Transmission Transfer Case System is not to factory specifications.

OBDII Code P1866 - Transmission Transfer Case System Concern Servicing Required - AutoCodes.com — P1866 Code - Transmission Transfer Case System Concern Servicing Required

What does this mean?

What are the Possible Causes of the DTC P1866?

Faulty Transfer Case Shift Motor
Transfer Case Shift Motor harness is open or shorted
Transfer Case Shift Motor circuit poor electrical connection

How to Fix the DTC P1866?

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 P1866 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 P1866 code?

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Need Help with the Code?

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Information for Specific Makes

P1866 Ford — Transmission Transfer Case System Concern Servicing Required
P1866 Lincoln — Transmission Transfer Case System Concern Servicing Required
P1866 Mercury — Transmission Transfer Case System Concern Servicing Required
P1866 Mitsubishi — High Side 2 System Overcurrent

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.