AutoCodes – GMC – P18C1 2018 GMC Sierra

P18C1 2018 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 2018 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 1,624

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 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 C signal circuit has an invalid voltage range of 0.70 – 0.96 V, for 2.7 s.

OBDII Code P18C1 2018 GMC Sierra - Internal Mode Switch 'C' Circuit Stuck On Invalid Voltage Range - AutoCodes.com — P18C1 2018 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 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 P18C1 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 P18C1 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 P18C1 2018 GMC Sierra code?

Need more information on how to fix the P18C1 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 P18C1 2018 GMC Sierra Code

1. What are the common symptoms of OBDII code P18C1 in a 2018 GMC Sierra?

- Common symptoms include issues with shifting gears, inability to start the vehicle, and the transmission being stuck in a single gear.

2. What causes the Internal Mode Switch 'C' Circuit Stuck On Invalid Voltage Range in a 2018 GMC Sierra?

- The most common causes of this OBDII code are a faulty internal mode switch, damaged wiring or connectors, or issues with the transmission control module.

3. How can I diagnose the Internal Mode Switch 'C' Circuit issue in my 2018 GMC Sierra?

- To diagnose this problem, you can use a diagnostic scanner to retrieve the OBDII code P18C1 and perform a thorough inspection of the internal mode switch and related wiring.

4. Can I still drive my 2018 GMC Sierra with the Internal Mode Switch 'C' Circuit issue?

- It is not recommended to drive the vehicle with this issue as it may lead to further damage to the transmission system. It's best to address the problem promptly.

5. How do I repair OBDII code P18C1 in my 2018 GMC Sierra?

- Repairing this issue typically involves replacing the faulty internal mode switch, repairing or replacing damaged wiring, or addressing any issues with the transmission control module.

6. Is it possible to reset the OBDII code P18C1 in a 2018 GMC Sierra without making repairs?

- While you can clear the code temporarily, it is crucial to address the underlying issue causing the code to prevent further transmission problems.

7. How much does it cost to fix the Internal Mode Switch 'C' Circuit issue in a 2018 GMC Sierra?

- The cost of repairing this issue can vary depending on the specific cause of the problem, but it may range from a few hundred to over a thousand dollars, including parts and labor.

8. Can a DIY enthusiast fix the OBDII code P18C1 in a 2018 GMC Sierra?

- While some DIY enthusiasts may have the skills to address this issue, it is recommended to consult a professional mechanic due to the complexity of transmission system repairs.

9. Will disconnecting the battery reset the OBDII code P18C1 in my 2018 GMC Sierra?

- Disconnecting the battery may reset the code temporarily, but it is not a long-term solution. The underlying issue must be fixed to prevent the code from reappearing.

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

- The repair time can vary depending on the extent of the problem and the availability of parts, but it may take a few hours to a full day to address this issue properly.