AutoCodes – GMC – P18B9 GMC

P18B9 GMC Code - Internal Mode Switch 'S' 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: P18B9 GMC indicates Internal Mode Switch 'S' 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 2,417

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 P18B9 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 S signal circuit voltage is between 0.966–1.68 V, for 1.75 s.

OBDII Code P18B9 GMC - Internal Mode Switch 'S' Circuit Shorted - AutoCodes.com — P18B9 GMC Code - Internal Mode Switch 'S' Circuit Shorted

What does this mean?

What are the Possible Causes of the DTC P18B9 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 P18B9 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 P18B9 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 P18B9 GMC code?

Need more information on how to fix the P18B9 GMC code? Get Access to Factory Service Manuals

Need Help with the Code?

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Information for Specific GMC Models

P18B9 2014 GMC Sierra — Internal Mode Switch 'S' Circuit Shorted
P18B9 2015 GMC Sierra — Internal Mode Switch 'S' Circuit Shorted
P18B9 2016 GMC Sierra — Internal Mode Switch 'S' Circuit Shorted
P18B9 2017 GMC Sierra — Internal Mode Switch 'S' Circuit Shorted
P18B9 2018 GMC Sierra — Internal Mode Switch 'S' 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 P18B9 GMC Code

1. What are the common symptoms of OBDII code P18B9 GMC (Internal Mode Switch 'S' Circuit Shorted)?

The common symptoms include issues with shifting gears, hesitation during acceleration, and the transmission staying in one gear.

2. What causes the OBDII code P18B9 GMC to trigger?

This code is typically triggered by a short circuit in the 'S' circuit of the internal mode switch, which is responsible for providing transmission gear position information to the vehicle's computer.

3. How serious is the OBDII code P18B9 GMC?

This code can lead to drivability issues and potentially cause further damage to the transmission if left unresolved. It is recommended to address the issue promptly.

4. How can I diagnose the Internal Mode Switch 'S' Circuit Shorted issue?

Diagnosing this issue usually involves checking the wiring and connections related to the internal mode switch, as well as using a scan tool to analyze the data and pinpoint the exact location of the short circuit.

5. Can I drive my vehicle with the P18B9 GMC code present?

While it may be possible to drive the vehicle with this code present, it is not recommended as it can lead to transmission problems and potentially leave you stranded.

6. How can I repair the Internal Mode Switch 'S' Circuit Shorted issue?

Repairing this issue often involves replacing the internal mode switch, fixing any damaged wiring or connections, and clearing the code from the vehicle's computer with a scan tool.

7. Is it possible to prevent the OBDII code P18B9 GMC from reoccurring?

Ensuring proper maintenance of the transmission system, including regular inspections and addressing any issues promptly, can help prevent this code from reoccurring.

8. Can a DIY enthusiast fix the Internal Mode Switch 'S' Circuit Shorted problem?

While some DIY enthusiasts may have the skills to address this issue, it is recommended to consult a professional mechanic with experience in diagnosing and repairing transmission-related problems.

9. How much does it cost to fix the OBDII code P18B9 GMC?

The cost of fixing this issue can vary depending on the extent of the damage, the cost of parts, and the labor rates of the repair shop. It is advisable to obtain quotes from different mechanics.

10. Will resetting the OBDII code P18B9 GMC make it go away permanently?

Simply resetting the code without addressing the underlying issue will not make it go away permanently. It is crucial to diagnose and repair the root cause of the problem to prevent the code from reoccurring.