AutoCodes – GMC – P18C1 2017 GMC Sierra

P18C1 2017 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 2017 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 214

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 2017 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 2017 GMC Sierra - Internal Mode Switch 'C' Circuit Stuck On Invalid Voltage Range - AutoCodes.com — P18C1 2017 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 2017 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 2017 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 2017 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 2017 GMC Sierra code?

Need more information on how to fix the P18C1 2017 GMC Sierra code? Get Access to Factory Service Manuals

Need Help with the Code?

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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 2017 GMC Sierra Code

1. What are common symptoms of the OBDII code P18C1 in a 2017 GMC Sierra?

The common symptoms of the P18C1 code include issues with shifting gears, erratic transmission behavior, and the illumination of the check engine light on the dashboard.

2. What could cause the Internal Mode Switch 'C' Circuit to be stuck on an invalid voltage range in a 2017 GMC Sierra?

Possible causes for this issue include a faulty internal mode switch, damaged wiring or connectors in the circuit, or a malfunctioning transmission control module.

3. How can I diagnose the Internal Mode Switch 'C' Circuit issue in my 2017 GMC Sierra?

To diagnose the problem, a technician can use a scan tool to read the OBDII code P18C1, inspect the internal mode switch and its connections, perform a voltage test on the circuit, and check for any related transmission issues.

4. Can I continue driving my 2017 GMC Sierra with the P18C1 code present?

It is not recommended to continue driving with the P18C1 code active, as it can lead to further transmission damage and unsafe driving conditions. It is best to address the issue promptly.

5. How can I repair the Internal Mode Switch 'C' Circuit stuck on an invalid voltage range in my 2017 GMC Sierra?

Repair options may include replacing the internal mode switch, repairing or replacing damaged wiring or connectors, resetting the transmission control module, or seeking professional help from a qualified mechanic.

6. Will resetting the check engine light resolve the P18C1 code in my 2017 GMC Sierra?

Simply resetting the check engine light will not fix the underlying issue causing the P18C1 code. It is important to diagnose the problem and address it accordingly to prevent further complications.

7. Are there any DIY tips for troubleshooting the P18C1 code in a 2017 GMC Sierra?

While basic inspections like checking for loose connections or damaged wiring can be done by DIY enthusiasts, diagnosing and repairing the Internal Mode Switch 'C' Circuit issue may require specialized tools and expertise best handled by a professional.

8. How much does it typically cost to fix the P18C1 code in a 2017 GMC Sierra?

The cost of repairing the P18C1 code can vary depending on the specific cause of the issue, parts needed for replacement, and labor costs. It is advisable to obtain quotes from reputable auto repair shops for accurate pricing.

9. Can extreme weather conditions affect the Internal Mode Switch 'C' Circuit in a 2017 GMC Sierra?

Extreme weather conditions can potentially impact the internal mode switch and related components, leading to issues like the circuit being stuck on an invalid voltage range. Proper maintenance and care can help prevent weather-related damage.

10. Is the P18C1 code a common issue in 2017 GMC Sierra vehicles?

While the P18C1 code may not be as common as other OBDII codes, it can still occur in 2017 GMC Sierra vehicles due to various factors such as wear and tear, electrical issues, or component failure.