AutoCodes – GMC – U1521 GMC

U1521 GMC Code - Headlamp Control Module/Headlamp Leveling Control Module Lost Communication With Left Headlamp On LIN Bus

Quick Answer

The Local Interconnect Network (LIN) Bus consists of a single wire with a transmission rate of 10.417 Kbit/s. Common causes include Faulty Left Headlamp, Left Headlamp harness is open or shorted, Left Headlamp 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: U1521 GMC indicates Headlamp Control Module/Headlamp Leveling Control Module Lost Communication With Left Headlamp On LIN Bus.
Common causes: Common causes include Faulty Left Headlamp, Left Headlamp harness is open or shorted, Left Headlamp 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 590

How popular is this code, among other codes? Low

Repair Importance Level

Urgent (Diagnose now) (3 / 3)

Repair Difficulty Level

Advanced (Pro) (3 / 3)

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Table of contents Created and updated by William Orellana on Jun 03, 2020

Description
Possible Causes
How To Fix
Cost to Diagnose
FAQs
Comments
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Code U1521 GMC Description

The Local Interconnect Network (LIN) Bus consists of a single wire with a transmission rate of 10.417 Kbit/s. The serial data is transmitted over a LIN circuit between a master control module and other LIN devices within a particular subsystem. If serial data communication is lost between any of the LIN devices on the LIN bus network, the master control module will set a no communication code against the non-communicating LIN device. The Diagnostic Trouble Code (DTC) will set when a supervised periodic message to the Headlamp Control Module/Headlamp Leveling Control Module from the Left Headlamp has not been received.

OBDII Code U1521 GMC - Headlamp Control Module/Headlamp Leveling Control Module Lost Communication With Left Headlamp On LIN Bus - AutoCodes.com — U1521 GMC Code - Headlamp Control Module/Headlamp Leveling Control Module Lost Communication With Left Headlamp On LIN Bus

What does this mean?

What are the Possible Causes of the DTC U1521 GMC?

Faulty Left Headlamp
Left Headlamp harness is open or shorted
Left Headlamp circuit poor electrical connection
Faulty Headlamp Control Module/Headlamp Leveling Control Module

How to Fix the DTC U1521 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 is the Cost to Diagnose the Code?

Labor: 1.0

To diagnose the U1521 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.

Frequently Asked Questions about U1521 GMC Code

1. What are the common symptoms of OBDII code U1521 in a GMC vehicle?

- Common symptoms include malfunctioning left headlamp, flickering lights, or complete loss of control over the headlamp's movement.

2. What does the U1521 code indicate in a GMC vehicle?

- The U1521 code indicates that the headlamp control module or headlamp leveling control module has lost communication with the left headlamp on the LIN bus.

3. What are the possible causes of the U1521 code in a GMC vehicle?

- Possible causes include a faulty headlamp control module, damaged wiring or connectors, a malfunctioning headlamp leveling control module, or issues with the LIN bus communication network.

4. How can I diagnose the U1521 code in my GMC vehicle?

- Diagnosing the U1521 code typically involves using a diagnostic scanner to retrieve the specific fault codes, checking the wiring and connectors for any visible damage, and testing the headlamp control module and headlamp leveling control module for proper functionality.

5. Can I still drive my GMC vehicle with the U1521 code present?

- It is not recommended to drive your vehicle with the U1521 code present, as it can affect the proper functioning of the left headlamp, compromising visibility and safety while driving.

6. How can I repair the U1521 code in my GMC vehicle?

- Repairing the U1521 code may involve replacing the faulty headlamp control module or headlamp leveling control module, repairing or replacing damaged wiring or connectors, and ensuring proper communication on the LIN bus network.

7. Is the U1521 code a common issue in GMC vehicles?

- The U1521 code related to headlamp control module communication issues is not extremely common but can occur in GMC vehicles with advanced lighting systems.

8. Can a DIY enthusiast fix the U1521 code issue in a GMC vehicle?

- While some basic troubleshooting steps can be performed by a DIY enthusiast, diagnosing and repairing the U1521 code may require advanced knowledge and specialized tools, so it is recommended to consult a professional mechanic.

9. Will resetting the U1521 code make it disappear permanently?

- Resetting the U1521 code may temporarily clear the warning light, but if the underlying issue causing the code is not addressed, it is likely to reappear once the system detects the problem again.

10. How much can I expect to pay to fix the U1521 code in a GMC vehicle?

- The cost to fix the U1521 code can vary depending on the specific cause of the issue, but it may range from a few hundred to over a thousand dollars, including parts and labor.