AutoCodes – GMC – P18C3 GMC

P18C3 GMC 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 GMC 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 985

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

What does this mean?

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

Need more information on how to fix the P18C3 GMC code? Get Access to Factory Service Manuals

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Information for Specific GMC Models

P18C3 2014 GMC Sierra — Internal Mode Switch 'S' Circuit Stuck On Invalid Voltage Range
P18C3 2015 GMC Sierra — Internal Mode Switch 'S' Circuit Stuck On Invalid Voltage Range
P18C3 2016 GMC Sierra — Internal Mode Switch 'S' Circuit Stuck On Invalid Voltage Range
P18C3 2017 GMC Sierra — Internal Mode Switch 'S' Circuit Stuck On Invalid Voltage Range
P18C3 2018 GMC Sierra — Internal Mode Switch 'S' Circuit Stuck On Invalid Voltage Range

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 GMC Code

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

The common symptoms of OBDII code P18C3 include issues with shifting gears, difficulty engaging or disengaging gears, and the transmission not responding properly.

2. What can cause the Internal Mode Switch 'S' Circuit to be stuck on an invalid voltage range in a GMC vehicle?

Possible causes of the Internal Mode Switch 'S' Circuit being stuck on an invalid voltage range include a faulty internal mode switch, damaged wiring or connectors, or a problem with the transmission control module.

3. How can I diagnose the Internal Mode Switch 'S' Circuit issue in my GMC vehicle?

Diagnosing the Internal Mode Switch 'S' Circuit issue typically involves using a scan tool to read the OBDII trouble code, checking the wiring and connectors for any damage, and testing the internal mode switch for proper functionality.

4. Can driving with OBDII code P18C3 cause further damage to my GMC vehicle?

Continuing to drive with OBDII code P18C3 can potentially cause further damage to the transmission system, leading to more severe issues over time. It is recommended to address the problem promptly.

5. Is it safe to continue driving my GMC vehicle with the Internal Mode Switch 'S' Circuit stuck on an invalid voltage range?

It is not recommended to continue driving your GMC vehicle with the Internal Mode Switch 'S' Circuit stuck on an invalid voltage range, as it can affect the performance and safety of the vehicle. It is best to have the issue diagnosed and repaired as soon as possible.

6. How can I repair the Internal Mode Switch 'S' Circuit issue in my GMC vehicle?

Repairing the Internal Mode Switch 'S' Circuit issue may involve replacing the internal mode switch, repairing or replacing damaged wiring or connectors, or reprogramming the transmission control module. It is advisable to consult a professional mechanic for proper diagnosis and repair.

7. Can a DIY enthusiast fix the Internal Mode Switch 'S' Circuit problem in a GMC vehicle?

While some DIY enthusiasts may have the skills to diagnose and repair the Internal Mode Switch 'S' Circuit problem, it is a complex issue that may require specialized tools and expertise. It is recommended to seek assistance from a qualified mechanic for proper diagnosis and repair.

8. How much does it cost to repair OBDII code P18C3 in a GMC vehicle?

The cost of repairing OBDII code P18C3 can vary depending on the underlying cause of the issue, the extent of the damage, and the labor rates in your area. It is advisable to obtain quotes from different mechanics to get an accurate estimate.

9. Can resetting the OBDII code P18C3 clear the issue in a GMC vehicle?

Resetting the OBDII code P18C3 may temporarily clear the warning light, but it will not fix the underlying problem causing the Internal Mode Switch 'S' Circuit issue. It is important to diagnose and repair the issue to prevent further complications.

10. How can I prevent OBDII code P18C3 from occurring in my GMC vehicle in the future?

To prevent OBDII code P18C3 from occurring in the future, you can perform regular maintenance on your vehicle, including checking the transmission system for any signs of wear or damage, and addressing any issues promptly to avoid further complications.