AutoCodes – GMC – U18C8 GMC

U18C8 GMC Code - Glow Plug Control Module Lost Communication with Engine Control Module

Quick Answer

Devices connected to the serial data circuits monitor for serial data communications during normal vehicle operation. Common causes include Faulty Engine Control Module (ECM), Engine Control Module harness is open or shorted, Engine 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: U18C8 GMC indicates Glow Plug Control Module Lost Communication with Engine Control Module.
Common causes: Common causes include Faulty Engine Control Module (ECM), Engine Control Module harness is open or shorted, Engine 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 191

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 Jul 24, 2023

Description
Possible Causes
How To Fix
Cost to Diagnose
FAQs
Comments
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Code U18C8 GMC Description

Devices connected to the serial data circuits monitor for serial data communications during normal vehicle operation. Operating information and commands are exchanged among the devices. The devices 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 device as an availability indication of the transmitter device. Each message contains the identification number of the transmitter device. The Diagnostic Trouble Code (DTC) will be set when a supervised periodic message that includes the transmitter device availability has not been received.

OBDII Code U18C8 GMC - Glow Plug Control Module Lost Communication with Engine Control Module - AutoCodes.com — U18C8 GMC Code - Glow Plug Control Module Lost Communication with Engine Control Module

What does this mean?

What are the Possible Causes of the DTC U18C8 GMC?

Faulty Engine Control Module (ECM)
Engine Control Module harness is open or shorted
Engine Control Module circuit poor electrical connection
Faulty Body Control Module (BCM)

How to Fix the DTC U18C8 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 U18C8 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.

Need more information about the U18C8 GMC code?

Need more information on how to fix the U18C8 GMC code? Get Access to Factory Service Manuals

Need Help with the Code?

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Information for Specific GMC Models

U18C8 2014 GMC Sierra — Glow Plug Control Module Lost Communication with Engine Control Module
U18C8 2015 GMC Sierra — Glow Plug Control Module Lost Communication with Engine Control Module
U18C8 2016 GMC Sierra — Glow Plug Control Module Lost Communication with Engine Control Module
U18C8 2017 GMC Sierra — Glow Plug Control Module Lost Communication with Engine Control Module
U18C8 2018 GMC Sierra — Glow Plug Control Module Lost Communication with Engine 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 U18C8 GMC Code

What does OBDII code U18C8 mean on a GMC vehicle?

Code U18C8 indicates that the Glow Plug Control Module has lost communication with the Engine Control Module (ECM), which may affect the glow plug system's performance in diesel engines.

What are the common symptoms of code U18C8 on a GMC?

Symptoms may include a check engine light, difficulty starting the engine (especially in cold weather), poor engine performance, or a lack of pre-glow operation.

What causes code U18C8 to appear on a GMC?

Possible causes include a faulty Glow Plug Control Module, a malfunctioning ECM, damaged or corroded wiring, poor electrical connections, or software issues.

Can a weak battery cause the U18C8 code?

Yes, low battery voltage or an unstable electrical system can disrupt communication between modules and trigger the U18C8 code.

How can I diagnose the U18C8 code on my GMC?

Start by checking for loose or corroded wiring between the Glow Plug Control Module and the ECM. Use a scan tool to verify module communication and inspect voltage levels.

Is it safe to drive with a U18C8 code active?

Driving may be possible, but the engine could experience starting issues or reduced performance, especially in cold conditions. Prolonged driving without repair may cause further damage.

How do I fix the U18C8 code on a GMC vehicle?

Repairs may involve inspecting and repairing wiring or connectors, replacing the Glow Plug Control Module, updating the ECM software, or replacing a faulty ECM.

Can a software update fix the U18C8 code?

In some cases, a software update for the ECM or Glow Plug Control Module can resolve communication issues and clear the code.

Does the U18C8 code apply to all GMC vehicles?

The U18C8 code is specific to GMC vehicles equipped with diesel engines that use a Glow Plug Control Module. It may not appear on gasoline-powered models.

Will clearing the U18C8 code with a scan tool fix the problem?

Clearing the code may temporarily remove the warning light, but it wonâ€™t fix the underlying issue. The code will likely return if the root cause isnâ€™t addressed.