AutoCodes – Generic – U1804

U1804 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 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 1,537

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

What does this mean?

What are the Possible Causes of the DTC U1804?

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?

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 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 U1804 code?

Need more information on how to fix the U1804 code? Get Access to Factory Service Manuals

Need Help with the Code?

Get the U1804 code diagnosed by a professional: Find a repair shop in your area

Information for Specific Makes

U1804 Buick — Lost Communication With Inside Rearview Mirror Control Module
U1804 Cadillac — Lost Communication With Inside Rearview Mirror Control Module
U1804 Chevrolet — Lost Communication With Inside Rearview Mirror Control Module
U1804 GMC — Lost Communication With Inside Rearview Mirror Control Module

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 Code

1. What are common symptoms of OBDII code U1804?

- Common symptoms include the interior rearview mirror not functioning properly, loss of communication with the mirror control module, and potential issues with other vehicle electronics.

2. What are possible causes of OBDII code U1804?

- Possible causes include a faulty inside rearview mirror control module, damaged wiring or connectors, a blown fuse, or issues with the vehicle's main computer system.

3. How can I diagnose OBDII code U1804?

- Diagnosis typically involves checking the wiring and connectors leading to the inside rearview mirror control module, inspecting the fuse related to the module, and using a scan tool to communicate with the module.

4. Can OBDII code U1804 be fixed without professional help?

- In some cases, simple issues like a blown fuse or loose connector can be addressed without professional help. However, more complex issues may require the expertise of a mechanic or auto technician.

5. Is it safe to drive with OBDII code U1804 present?

- It is generally safe to drive with this code, but it is recommended to address the underlying issue as soon as possible to prevent potential problems with other vehicle systems.

6. How much does it cost to repair OBDII code U1804?

- The cost of repair can vary depending on the specific cause of the code. Simple fixes like replacing a fuse or connector may be relatively inexpensive, while replacing the control module could be more costly.

7. Can resetting the OBDII system clear code U1804?

- Resetting the OBDII system may temporarily clear the code, but if the underlying issue is not addressed, the code is likely to reappear.

8. Are there any DIY repair tips for OBDII code U1804?

- DIY repair tips may include checking and replacing fuses, inspecting wiring for damage, and ensuring connectors are securely attached. However, complex diagnostics and repairs are best left to professionals.

9. Will OBDII code U1804 cause a vehicle to fail an emissions test?

- The impact on emissions testing will depend on the regulations in your area. It's best to address the code promptly to ensure your vehicle meets emissions requirements.

10. Can extreme weather conditions trigger OBDII code U1804?

- Extreme weather conditions, such as extreme heat or cold, may contribute to electrical issues that could trigger code U1804. Insulating or protecting wiring and components may help prevent weather-related problems.