AutoCodes – GMC – P18C1 2014 GMC Sierra

P18C1 2014 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 2014 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 169

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 2014 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 2014 GMC Sierra - Internal Mode Switch 'C' Circuit Stuck On Invalid Voltage Range - AutoCodes.com — P18C1 2014 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 2014 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 2014 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 2014 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 2014 GMC Sierra code?

Need more information on how to fix the P18C1 2014 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 2014 GMC Sierra Code

1. What are the common symptoms of OBDII code P18C1 in a 2014 GMC Sierra?

- Common symptoms include issues with shifting gears, the transmission staying in one gear, difficulty starting the vehicle, and the check engine light illuminating.

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

- This code indicates a problem with the circuit that controls the transmission's internal mode switch, specifically that it is stuck in an invalid voltage range.

3. What are the possible causes of the Internal Mode Switch 'C' Circuit Stuck On Invalid Voltage Range code?

- Possible causes include a faulty internal mode switch, damaged wiring or connectors, a malfunctioning transmission control module, or issues with the vehicle's battery or charging system.

4. How can I diagnose the cause of OBDII code P18C1 in my 2014 GMC Sierra?

- To diagnose this code, you may need to perform a thorough inspection of the internal mode switch, check the wiring and connectors for damage, and use a diagnostic tool to scan the vehicle's computer for specific error codes.

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

- It is not recommended to drive your vehicle with this code present, as it can lead to further damage to the transmission system. It is best to address the issue promptly to avoid more severe problems.

6. Is it possible to reset the OBDII code P18C1 without repairing the underlying issue?

- Resetting the code without addressing the underlying issue will only temporarily turn off the check engine light. The problem will persist, and the code is likely to return if the root cause is not fixed.

7. How much does it typically cost to repair OBDII code P18C1 in a 2014 GMC Sierra?

- The cost of repairing this code can vary depending on the specific cause of the issue. It is recommended to consult a professional mechanic for an accurate diagnosis and cost estimate.

8. Can I repair the Internal Mode Switch 'C' Circuit issue myself, or should I seek professional help?

- Repairing this issue may require advanced knowledge of the vehicle's transmission system and specialized tools. It is usually best to seek the assistance of a qualified mechanic for proper diagnosis and repair.

9. Are there any preventative maintenance steps I can take to avoid encountering the P18C1 code in the future?

- Regularly inspecting and maintaining your vehicle's transmission system, checking for any signs of wear or damage, and addressing any issues promptly can help prevent the occurrence of this code.

10. How long does it typically take to repair the Internal Mode Switch 'C' Circuit issue in a 2014 GMC Sierra?

- The repair time can vary depending on the specific cause of the problem and the availability of parts. It is advisable to consult with a mechanic to get a better estimate of the repair timeline.