AutoCodes – GMC – P18C1 2015 GMC Sierra

P18C1 2015 GMC Sierra Code - Internal Mode Switch 'C' 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: P18C1 2015 GMC Sierra indicates Internal Mode Switch 'C' 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 188

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 P18C1 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 the internal mode switch C signal circuit has an invalid voltage range of 0.70 – 0.96 V, for 2.7 s.

OBDII Code P18C1 2015 GMC Sierra - Internal Mode Switch 'C' Circuit Stuck On Invalid Voltage Range - AutoCodes.com — P18C1 2015 GMC Sierra Code - Internal Mode Switch 'C' Circuit Stuck On Invalid Voltage Range

What does this mean?

What are the Possible Causes of the DTC P18C1 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 P18C1 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 P18C1 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 P18C1 2015 GMC Sierra code?

Need more information on how to fix the P18C1 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 P18C1 2015 GMC Sierra Code

1. What are the common symptoms of OBDII code P18C1 in a 2015 GMC Sierra?

Symptoms may include issues with shifting gears, the inability to start the vehicle, the check engine light being illuminated, and potential transmission problems.

2. What can cause the Internal Mode Switch 'C' Circuit to be stuck on an invalid voltage range in a 2015 GMC Sierra?

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

3. How can I diagnose the Internal Mode Switch 'C' Circuit issue in my 2015 GMC Sierra?

Diagnosis typically involves using a scan tool to retrieve the OBDII code P18C1, checking the wiring and connectors for damage, and testing the internal mode switch for proper functionality.

4. Can I continue driving my 2015 GMC Sierra with OBDII code P18C1?

It is not recommended to continue driving the vehicle with this code present, as it can lead to further transmission issues and potential safety risks.

5. What are the repair options for fixing the Internal Mode Switch 'C' Circuit issue in a 2015 GMC Sierra?

Repairs may involve replacing the internal mode switch, repairing or replacing damaged wiring or connectors, or reprogramming the transmission control module.

6. How much does it typically cost to repair OBDII code P18C1 in a 2015 GMC Sierra?

Costs can vary depending on the specific cause of the issue, but repairs for this code may range from $200 to $500 or more, including parts and labor.

7. Can I reset the OBDII code P18C1 in my 2015 GMC Sierra without making any repairs?

While you can clear the code using a scan tool, it is not recommended to do so without addressing the underlying issue, as it may lead to recurring problems.

8. How long does it take to repair the Internal Mode Switch 'C' Circuit issue in a 2015 GMC Sierra?

The repair time can vary depending on the exact cause of the problem and the complexity of the repair, but it may take a few hours to a full day to complete.

9. Are there any preventive measures to avoid OBDII code P18C1 in a 2015 GMC Sierra?

Regular maintenance, such as checking the transmission system and wiring for any signs of wear or damage, can help prevent issues that may trigger this code.

10. Can a DIY enthusiast fix the Internal Mode Switch 'C' Circuit problem in a 2015 GMC Sierra?

While some experienced DIY enthusiasts may be able to diagnose and repair simple wiring or connector issues related to this code, it is recommended to consult a professional for proper diagnosis and repair of internal components.