AutoCodes – GMC – P18C3 2018 GMC Sierra

P18C3 2018 GMC Sierra Code - Internal Mode Switch 'S' Circuit Stuck On Invalid Voltage Range

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: P18C3 2018 GMC Sierra indicates Internal Mode Switch 'S' Circuit Stuck On Invalid Voltage Range.
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 126

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 P18C3 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 has an internal mode switch S signal circuit has an invalid voltage range of 0.70 – 0.96 V, for 2.7 s.

OBDII Code P18C3 2018 GMC Sierra - Internal Mode Switch 'S' Circuit Stuck On Invalid Voltage Range - AutoCodes.com — P18C3 2018 GMC Sierra Code - Internal Mode Switch 'S' Circuit Stuck On Invalid Voltage Range

What does this mean?

What are the Possible Causes of the DTC P18C3 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 P18C3 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 P18C3 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 P18C3 2018 GMC Sierra code?

Need more information on how to fix the P18C3 2018 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 P18C3 2018 GMC Sierra Code

1. What are the common symptoms of OBDII code P18C3 in a 2018 GMC Sierra?

- Common symptoms include issues with shifting gears, the vehicle not starting, and the transmission being stuck in a single gear.

2. What does the OBDII code P18C3 ('Internal Mode Switch 'S' Circuit Stuck On Invalid Voltage Range') indicate in a 2018 GMC Sierra?

- This code typically indicates a problem with the internal mode switch, specifically the 'S' circuit, where the voltage range is outside the acceptable parameters.

3. What are the possible causes of OBDII code P18C3 in a 2018 GMC Sierra?

- Potential causes include a faulty internal mode switch, damaged wiring or connectors related to the 'S' circuit, or issues with the transmission control module.

4. How serious is OBDII code P18C3 in a 2018 GMC Sierra?

- This code can lead to significant transmission issues and affect the drivability of the vehicle, so it should be addressed promptly to prevent further damage.

5. Can I continue driving my 2018 GMC Sierra with OBDII code P18C3 present?

- It is not recommended to drive the vehicle with this code active, as it can cause transmission problems and potentially leave you stranded on the road.

6. How can I diagnose OBDII code P18C3 in my 2018 GMC Sierra?

- Diagnosing this code typically involves using a scan tool to retrieve the fault codes, checking the internal mode switch and related components for faults, and testing the circuit for proper voltage.

7. What are some repair tips for fixing OBDII code P18C3 in a 2018 GMC Sierra?

- Repair tips may include replacing the internal mode switch, repairing any damaged wiring or connectors, and resetting the transmission control module after the issue is resolved.

8. Can I fix OBDII code P18C3 in my 2018 GMC Sierra myself, or do I need a professional mechanic?

- While some DIY enthusiasts may be able to address this code themselves, it is recommended to consult a professional mechanic with experience in transmission systems for proper diagnosis and repair.

9. How much does it typically cost to repair OBDII code P18C3 in a 2018 GMC Sierra?

- The cost of repairs can vary depending on the specific issue causing the code, but it may involve parts replacement, labor costs, and potential diagnostic fees from a mechanic.

10. Are there any preventive measures to avoid OBDII code P18C3 from occurring in my 2018 GMC Sierra in the future?

- Regular maintenance, such as checking the transmission system for any signs of wear or damage, can help prevent issues like OBDII code P18C3 from occurring.