AutoCodes – GMC – P18C1 2016 GMC Sierra

P18C1 2016 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 2016 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.

Save to My Repairs My Repairs

AI answers can be wrong. Verify repairs with testing. Open full chat

Code Popularity 167

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
Add Comment

💬 Need more help? Ask Coddy for instant answers — it's free!

Code P18C1 2016 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 2016 GMC Sierra - Internal Mode Switch 'C' Circuit Stuck On Invalid Voltage Range - AutoCodes.com — P18C1 2016 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 2016 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 2016 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 2016 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 2016 GMC Sierra code?

Need more information on how to fix the P18C1 2016 GMC Sierra code? Get Access to Factory Service Manuals

Need Help with the Code?

Get the P18C1 2016 GMC Sierra code diagnosed by a professional: Find a repair shop in your area

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 2016 GMC Sierra Code

1. What does the OBDII code P18C1 mean on a 2016 GMC Sierra?

P18C1 indicates an issue with the Internal Mode Switch 'C' Circuit, specifically that it is stuck on an invalid voltage range, which disrupts proper communication within the transmission system.

2. What are the common symptoms of code P18C1 in a 2016 GMC Sierra?

Symptoms may include the vehicle being stuck in one gear, erratic shifting, illumination of the check engine light, and reduced overall transmission performance.

3. What causes the P18C1 code to appear on a 2016 GMC Sierra?

The code can be caused by faulty wiring or connectors in the Internal Mode Switch 'C' Circuit, a malfunctioning internal mode switch, transmission control module (TCM) issues, or electrical short circuits.

4. Can I continue driving my 2016 GMC Sierra with the P18C1 code present?

It is not recommended to drive for extended periods with this code, as it may cause further damage to the transmission or affect vehicle safety due to shifting and performance issues.

5. How can I diagnose the P18C1 code on my GMC Sierra?

Diagnosis involves using an OBDII scanner to confirm the code, inspecting wiring and connectors for damage, testing the internal mode switch for proper operation, and checking the voltage range in the circuit.

6. How do I fix the P18C1 code on a 2016 GMC Sierra?

Repairs may include replacing damaged wiring or connectors, repairing or replacing the internal mode switch, and in some cases, reprogramming or replacing the transmission control module (TCM).

7. What tools are needed to fix the P18C1 code on a GMC Sierra?

Essential tools include an OBDII scanner, a multimeter to check voltage and continuity, and basic hand tools to inspect and repair wiring or replace components.

8. How much does it cost to repair the P18C1 code on a 2016 GMC Sierra?

Repair costs vary depending on the issue, but replacing the internal mode switch can range from $200 to $500, while wiring repairs may cost less. Dealer diagnostics and TCM replacement could increase costs.

9. Can a faulty internal mode switch cause other issues besides the P18C1 code?

Yes, a faulty internal mode switch can lead to transmission performance problems, difficulty shifting gears, or even failure to start in some cases.

10. Is the P18C1 code specific to GMC vehicles?

While P18C1 is commonly reported in GMC and Chevrolet vehicles, particularly trucks like the Sierra, it can appear in other vehicles sharing similar transmission systems.