AutoCodes – GMC – U1804 GMC

U1804 GMC Code - Lost Communication With Inside Rearview Mirror Control Module

Quick Answer

Modules connected to the Controller Area Network (CAN) serial data circuits monitor for serial data communications during normal vehicle operation. Common causes include Faulty Inside Rearview Mirror Control Module, Inside Rearview Mirror Control Module harness is open or shorted, Inside Rearview Mirror Control Module 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: U1804 GMC indicates Lost Communication With Inside Rearview Mirror Control Module.
Common causes: Common causes include Faulty Inside Rearview Mirror Control Module, Inside Rearview Mirror Control Module harness is open or shorted, Inside Rearview Mirror Control Module 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 482

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 07, 2020

Description
Possible Causes
How To Fix
Cost to Diagnose
FAQs
Comments
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Code U1804 GMC Description

Modules connected to the Controller Area Network (CAN) 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. The messages are supervised and also, some periodic messages are used by the receiver module as an availability indication of the transmitter module. Each message contains the identification number of the transmitter module. A module with a CAN serial data circuits malfunction or which loses power during the current ignition cycle will have a Loss of Communication Diagnostic Trouble Code (DTC) set by other modules that depend on information from that failed module. If any of the CAN circuit are shorted to battery voltage, ground, or between them no communication is possible on the CAN serial data bus, a Diagnostic Trouble Code will set.

OBDII Code U1804 GMC - Lost Communication With Inside Rearview Mirror Control Module - AutoCodes.com — U1804 GMC Code - Lost Communication With Inside Rearview Mirror Control Module

What does this mean?

What are the Possible Causes of the DTC U1804 GMC?

Faulty Inside Rearview Mirror Control Module
Inside Rearview Mirror Control Module harness is open or shorted
Inside Rearview Mirror Control Module circuit poor electrical connection

How to Fix the DTC U1804 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 U1804 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 U1804 GMC Code

What does the OBDII code U1804 mean for my GMC?

The U1804 code indicates a lost communication with the inside rearview mirror control module, which may affect features like auto-dimming and integrated safety systems.

What symptoms might I experience if my vehicle has the U1804 code?

Common symptoms include malfunctioning auto-dimming mirror features, issues with integrated safety systems, or warning lights related to the rearview mirror.

What are some common causes of the U1804 code?

Possible causes include a faulty inside rearview mirror control module, damaged wiring or connectors, or issues with the vehicle's communication network.

Can I drive my vehicle if the U1804 code is present?

While you can drive your vehicle, the functionality of the rearview mirror features may be compromised, and safety systems may not operate correctly.

How can I diagnose the U1804 code?

Start by checking the wiring and connectors related to the inside rearview mirror for damage or corrosion. Then, use an OBDII scanner to verify the code and check for any other related codes.

What repair options are available for the U1804 code?

Repairs may include replacing the inside rearview mirror control module, repairing damaged wiring, or addressing issues within the vehicle's communication network.

Is it necessary to reset the OBDII system after fixing the U1804 code?

Yes, after repairs, you should clear the code using an OBDII scanner and then run the vehicle to ensure the code does not reappear.

How can I prevent the U1804 code from recurring in the future?

Regularly inspect and maintain the wiring and connectors associated with the inside rearview mirror and ensure any electrical components are in good condition.

Could a software update fix the U1804 code?

Yes, sometimes a software update to the vehicleâ€™s control modules can resolve communication issues, so it's worth checking with a dealership or qualified technician.

What should I do if the U1804 code persists after repairs?

If the code persists, further diagnosis may be needed to check for deeper issues within the vehicle's communication network or other related control modules.