AutoCodes – GMC – P18B8 2015 GMC Sierra

P18B8 2015 GMC Sierra Code - Internal Mode Switch 'P' Circuit Shorted

Quick Answer

The internal mode switch indicates the Transmission Control Module (TCM) which gear position the vehicle operator has selected. Common causes include Faulty Transmission Internal Mode Switch, Transmission Internal Mode Switch harness is open or shorted, Transmission Internal Mode Switch 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: P18B8 2015 GMC Sierra indicates Internal Mode Switch 'P' Circuit Shorted.
Common causes: Common causes include Faulty Transmission Internal Mode Switch, Transmission Internal Mode Switch harness is open or shorted, Transmission Internal Mode Switch 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 177

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 Mar 23, 2024

Description
Possible Causes
How To Fix
Cost to Diagnose
FAQs
Comments
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Code P18B8 2015 GMC Sierra Description

The internal mode switch indicates the Transmission Control Module (TCM) which gear position the vehicle operator has selected. The internal mode switch consists of 5 separate hall effect switches. Each hall effect switch is supplied with a 9 V reference circuit and a signal circuit from the TCM. Each signal circuit for each gear selector position will have either a voltage reading of 0.70-0.96 V indicating On or 1.68-2.38 V indicating Off. The voltage values on each internal mode switch circuit will change and are dependent on the position of the gear selector. The state of each internal mode switch A/B/C/P/S signal circuit is displayed on the scan tool. The Diagnostic Trouble Code (DTC) will be set when the TCM has detected the internal mode switch P signal circuit voltage is between 0.966–1.68 V, for 1.75 s.

OBDII Code P18B8 2015 GMC Sierra - Internal Mode Switch 'P' Circuit Shorted - AutoCodes.com — P18B8 2015 GMC Sierra Code - Internal Mode Switch 'P' Circuit Shorted

What does this mean?

What are the Possible Causes of the DTC P18B8 2015 GMC Sierra?

Faulty Transmission Internal Mode Switch
Transmission Internal Mode Switch harness is open or shorted
Transmission Internal Mode Switch circuit poor electrical connection
Faulty Transmission Control Module (TCM)

How to Fix the DTC P18B8 2015 GMC Sierra?

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 P18B8 2015 GMC Sierra 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 P18B8 2015 GMC Sierra code?

Need more information on how to fix the P18B8 2015 GMC Sierra code? Get Access to Factory Service Manuals

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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 P18B8 2015 GMC Sierra Code

What are the common symptoms of OBDII code P18B8 in a 2015 GMC Sierra?

Common symptoms of OBDII code P18B8, Internal Mode Switch 'P' Circuit Shorted, in a 2015 GMC Sierra may include issues with shifting gears, erratic transmission behavior, and possibly triggering the vehicle into "limp mode."

What are the possible causes of OBDII code P18B8 in a 2015 GMC Sierra?

Potential causes of OBDII code P18B8 in a 2015 GMC Sierra include a faulty internal mode switch, damaged wiring or connectors related to the internal mode switch, or a malfunctioning transmission control module.

How can I diagnose OBDII code P18B8 in a 2015 GMC Sierra?

Diagnosing OBDII code P18B8 in a 2015 GMC Sierra typically involves using a scan tool to retrieve the trouble code, inspecting the internal mode switch and its circuitry for any visible damage, and testing the related components for proper functionality.

Can I continue to drive my 2015 GMC Sierra with OBDII code P18B8?

It is not recommended to continue driving a 2015 GMC Sierra with OBDII code P18B8 as it can lead to further damage to the transmission system. It is advisable to address the issue promptly to avoid potential safety risks and costly repairs.

What are some repair tips for resolving OBDII code P18B8 in a 2015 GMC Sierra?

Repair tips for fixing OBDII code P18B8 in a 2015 GMC Sierra may include checking and repairing any damaged wiring or connectors, replacing the internal mode switch if necessary, and resetting the trouble codes after completing the repairs.

Is it possible to reset OBDII code P18B8 in a 2015 GMC Sierra without fixing the underlying issue?

Resetting OBDII code P18B8 in a 2015 GMC Sierra without addressing the root cause is not recommended as the problem is likely to recur. It is crucial to diagnose and repair the issue causing the trouble code to prevent further complications.

How much does it typically cost to repair OBDII code P18B8 in a 2015 GMC Sierra?

The cost of repairing OBDII code P18B8 in a 2015 GMC Sierra can vary depending on the exact cause of the issue, labor rates, and the cost of parts. It is advisable to consult a professional mechanic for an accurate estimate.

Can a DIY enthusiast fix OBDII code P18B8 in a 2015 GMC Sierra?

While some DIY enthusiasts may be able to diagnose and repair OBDII code P18B8 in a 2015 GMC Sierra, it is recommended to have the necessary knowledge, tools, and experience to work on transmission-related issues. Otherwise, consulting a professional mechanic is advised.