AutoCodes – GMC – P18C3 2015 GMC Sierra

P18C3 2015 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 2015 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 144

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 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 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 2015 GMC Sierra - Internal Mode Switch 'S' Circuit Stuck On Invalid Voltage Range - AutoCodes.com — P18C3 2015 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 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 P18C3 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 P18C3 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 P18C3 2015 GMC Sierra code?

Need more information on how to fix the P18C3 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 P18C3 2015 GMC Sierra Code

1. What are common symptoms of OBDII code P18C3 on a 2015 GMC Sierra?

Common symptoms include the check engine light being illuminated, difficulty shifting gears, and the vehicle going into a "limp mode" where it operates at reduced performance.

2. What does the Internal Mode Switch 'S' Circuit Stuck On Invalid Voltage Range mean?

This code indicates that the electronic control module (ECM) has detected an issue with the internal mode switch circuit, specifically related to an invalid voltage range being detected.

3. What are possible causes of OBDII code P18C3 on a 2015 GMC Sierra?

Potential causes include a faulty internal mode switch, damaged wiring or connectors in the circuit, a malfunctioning ECM, or electrical issues within the vehicle's transmission system.

4. How serious is the Internal Mode Switch 'S' Circuit Stuck On Invalid Voltage Range code?

While the vehicle may still be operable with this code present, it is important to address it promptly to prevent potential transmission issues or further damage to the vehicle's components.

5. Can I continue driving my 2015 GMC Sierra with the P18C3 code?

It is not recommended to drive the vehicle extensively with this code present, as it can lead to transmission problems and potential safety hazards. It is best to have the issue diagnosed and repaired as soon as possible.

6. How can I diagnose and repair OBDII code P18C3 on my 2015 GMC Sierra?

Diagnosis typically involves using a scan tool to retrieve the trouble codes and inspecting the internal mode switch circuit for any visible damage. Repair may involve replacing the internal mode switch, repairing or replacing damaged wiring, or addressing any ECM issues.

7. Is it necessary to visit a mechanic for OBDII code P18C3 on a 2015 GMC Sierra?

While some DIY enthusiasts may attempt to diagnose and repair this code themselves, it is recommended to seek professional help, especially if you are not familiar with automotive electrical systems or transmission components.

8. Can resetting the ECM clear the P18C3 code on a 2015 GMC Sierra?

Resetting the ECM may temporarily clear the code, but if the underlying issue is not addressed, the code is likely to reappear. It's important to diagnose the problem and perform proper repairs to prevent recurrence.

9. How much does it typically cost to repair OBDII code P18C3 on a 2015 GMC Sierra?

The cost of repair can vary depending on the specific cause of the code, the labor rates in your area, and the parts needed for the repair. It's best to get a diagnostic from a mechanic to determine the exact cost.

10. Can neglecting the P18C3 code lead to further damage to my 2015 GMC Sierra?

Neglecting this code can potentially lead to more severe transmission issues over time, as well as decreased performance and fuel efficiency. It is best to address the problem promptly to avoid costly repairs in the future.