AutoCodes – GMC – P18B9 2018 GMC Sierra

P18B9 2018 GMC Sierra 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 2018 GMC Sierra 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.

Save to My Repairs My Repairs

AI answers can be wrong. Verify repairs with testing. Open full chat

Code Popularity 211

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
Add Comment

💬 Need more help? Ask Coddy for instant answers — it's free!

Code P18B9 2018 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 S signal circuit voltage is between 0.966–1.68 V, for 1.75 s.

OBDII Code P18B9 2018 GMC Sierra - Internal Mode Switch 'S' Circuit Shorted - AutoCodes.com — P18B9 2018 GMC Sierra Code - Internal Mode Switch 'S' Circuit Shorted

What does this mean?

What are the Possible Causes of the DTC P18B9 2018 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 P18B9 2018 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 P18B9 2018 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 P18B9 2018 GMC Sierra code?

Need more information on how to fix the P18B9 2018 GMC Sierra code? Get Access to Factory Service Manuals

Need Help with the Code?

Get the P18B9 2018 GMC Sierra code diagnosed by a professional: Find a repair shop in your area

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 2018 GMC Sierra Code

1. What are the common symptoms of OBDII code P18B9 in a 2018 GMC Sierra?

The common symptoms of OBDII code P18B9 related to the Internal Mode Switch 'S' Circuit Shorted in a 2018 GMC Sierra may include issues with the transmission shifting erratically or difficulty shifting gears.

2. What causes the Internal Mode Switch 'S' Circuit Shorted code in a 2018 GMC Sierra?

The OBDII code P18B9 in a 2018 GMC Sierra is typically triggered by a short circuit in the 'S' circuit of the internal mode switch, which may be caused by wiring issues, a faulty internal mode switch, or a damaged connector.

3. How serious is the Internal Mode Switch 'S' Circuit Shorted code in a 2018 GMC Sierra?

While the code may not necessarily cause immediate harm to the vehicle, ignoring it can lead to transmission problems and potential safety issues. It is recommended to address the issue promptly to ensure proper vehicle operation.

4. Can I continue driving my 2018 GMC Sierra with OBDII code P18B9?

It is generally not advisable to continue driving your 2018 GMC Sierra with the Internal Mode Switch 'S' Circuit Shorted code active, as it can lead to transmission issues or potential failures. It is recommended to have the vehicle inspected and repaired as soon as possible.

5. How can the Internal Mode Switch 'S' Circuit Shorted code in a 2018 GMC Sierra be diagnosed?

Diagnosing OBDII code P18B9 involves using a diagnostic scanner to retrieve the specific fault code, inspecting the internal mode switch and related wiring for any visible damage, and conducting further electrical testing to pinpoint the exact cause of the short circuit.

6. Is it possible to repair the Internal Mode Switch 'S' Circuit Shorted issue in a 2018 GMC Sierra at home?

Depending on your mechanical skills and access to tools, you may be able to address the Internal Mode Switch 'S' Circuit Shorted problem at home. However, due to the complexities of transmission systems, it is recommended to seek professional assistance for accurate diagnosis and repair.

7. How much does it typically cost to fix the OBDII code P18B9 in a 2018 GMC Sierra?

The cost of repairing the Internal Mode Switch 'S' Circuit Shorted code in a 2018 GMC Sierra can vary depending on the extent of the damage, parts needed for replacement, and labor costs. It is advisable to obtain quotes from reputable mechanics or dealerships for an accurate estimate.

8. Can resetting the OBDII system clear the P18B9 code in a 2018 GMC Sierra?

While clearing the OBDII system may temporarily remove the code, it will not fix the underlying issue causing the Internal Mode Switch 'S' Circuit Shorted problem. It is important to diagnose and address the root cause of the code to prevent further complications.

9. Will the OBDII code P18B9 cause my 2018 GMC Sierra to fail an emissions test?

If left unresolved, the OBDII code P18B9 indicating the Internal Mode Switch 'S' Circuit Shorted can lead to transmission malfunctions, which may affect the vehicle's emissions control system. It is recommended to address the issue to ensure compliance with emissions regulations.

10. How long does it typically take to repair the Internal Mode Switch 'S' Circuit Shorted issue in a 2018 GMC Sierra?

The repair time for OBDII code P18B9 in a 2018 GMC Sierra can vary depending on the exact cause of the problem, availability of parts, and the expertise of the technician. In general, the repair may take a few hours to a full day to complete.