AutoCodes – GMC – P18B8 GMC

P18B8 GMC 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 GMC 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 1,136

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

What does this mean?

What are the Possible Causes of the DTC P18B8 GMC?

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 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 P18B8 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 P18B8 GMC code?

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

Need Help with the Code?

Get the P18B8 GMC code diagnosed by a professional: Find a repair shop in your area

Information for Specific GMC Models

P18B8 2014 GMC Sierra — Internal Mode Switch 'P' Circuit Shorted
P18B8 2015 GMC Sierra — Internal Mode Switch 'P' Circuit Shorted
P18B8 2016 GMC Sierra — Internal Mode Switch 'P' Circuit Shorted
P18B8 2017 GMC Sierra — Internal Mode Switch 'P' Circuit Shorted
P18B8 2018 GMC Sierra — Internal Mode Switch 'P' Circuit Shorted

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 GMC Code

1. What are the common symptoms of OBDII code P18B8 GMC?

The common symptoms of code P18B8 GMC include issues with shifting gears, erratic transmission behavior, and possibly the transmission going into "limp mode."

2. What does the OBDII code P18B8 GMC indicate?

OBDII code P18B8 GMC indicates a problem with the Internal Mode Switch 'P' circuit being shorted in the transmission control module.

3. What are the possible causes of OBDII code P18B8 GMC?

Possible causes of code P18B8 GMC include a faulty internal mode switch, damaged wiring or connectors in the transmission control circuit, or a malfunctioning transmission control module.

4. How can I diagnose the Internal Mode Switch 'P' circuit shorted issue?

To diagnose the Internal Mode Switch 'P' circuit shorted problem, you can perform a visual inspection of the wiring and connectors, test the internal mode switch for proper operation, and use a scan tool to read the transmission control module for fault codes.

5. Can I continue driving my vehicle with OBDII code P18B8 GMC?

It is not recommended to continue driving your vehicle with OBDII code P18B8 GMC as it can lead to further transmission issues and potential safety hazards. It is best to address the problem promptly.

6. How can I repair OBDII code P18B8 GMC?

Repairing OBDII code P18B8 GMC may involve replacing the internal mode switch, repairing or replacing damaged wiring or connectors, or reprogramming the transmission control module. It is recommended to consult a professional mechanic for proper diagnosis and repair.

7. Will resetting the fault codes make OBDII code P18B8 GMC disappear?

Resetting the fault codes may temporarily clear the code, but if the underlying issue causing code P18B8 GMC is not addressed, the code is likely to reappear after some driving cycles.

8. Is OBDII code P18B8 GMC specific to certain GMC vehicle models?

OBDII code P18B8 GMC is specific to GMC vehicles and may be found in various models equipped with an Internal Mode Switch 'P' circuit in the transmission control system.

9. Can a DIY enthusiast fix OBDII code P18B8 GMC at home?

While some DIY enthusiasts may have the skills to diagnose and repair OBDII code P18B8 GMC, due to the complexity of transmission systems, it is recommended to seek professional help to ensure the issue is properly resolved.

10. How much does it typically cost to repair OBDII code P18B8 GMC at a repair shop?

The cost of repairing OBDII code P18B8 GMC at a repair shop can vary depending on the specific cause of the issue, the labor rates, and the cost of parts. It is advisable to obtain a few quotes from different repair shops for an accurate estimate.