AutoCodes – GMC – U0121 GMC

U0121 GMC Code - Lost Communication With Electronic Brake Control Module

Quick Answer

Modules connected to the high speed General Motor Local Area Network (GMLAN) serial data circuits monitor for serial data communications during normal vehicle operation. Common causes include Faulty Electronic Brake Control Module (EBCM), EBCM harness is open or shorted, EBCM 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: U0121 GMC indicates Lost Communication With Electronic Brake Control Module.
Common causes: Common causes include Faulty Electronic Brake Control Module (EBCM), EBCM harness is open or shorted, EBCM 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 23,539

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 Jun 29, 2020

Description
Possible Causes
How To Fix
Tech Notes
Cost to Diagnose
Comments
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Code U0121 GMC Description

Modules connected to the high speed General Motor Local Area Network (GMLAN) serial data circuits monitor for serial data communications during normal vehicle operation. Operating information and commands are exchanged among the modules. The modules have prerecorded information about what messages are needed to be exchanged on the serial data circuits, for each virtual network. The messages are supervised and also, some periodic messages are used by the receiver module as an availability indication of the transmitter module. The supervision time-out period is 250 ms. Each message contains the identification number of the transmitter module. The Diagnostic Trouble Code (DTC) sets when a periodic message from the Electronic Brake Control Module (EBCM) has not been received.

OBDII Code U0121 GMC - Lost Communication With Electronic Brake Control Module - AutoCodes.com — U0121 GMC Code - Lost Communication With Electronic Brake Control Module

What does this mean?

What are the Possible Causes of the DTC U0121 GMC?

Faulty Electronic Brake Control Module (EBCM)
EBCM harness is open or shorted
EBCM circuit poor electrical connection

How to Fix the DTC U0121 GMC?

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 does this mean?

Code Tech Notes

There is a factory service bulletin for the following GMC models:
2007-2010 GMC Sierra
2007-2010 GMC Yukon Models
2008-2010 GMC Sierra Hybrid
2008-2010 GMC Yukon Hybrid
GMC Factory Service Bulletin OBDII Code U0121

What is the Cost to Diagnose the Code?

Labor: 1.0

To diagnose the U0121 GMC 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.

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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.