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P1840 2016 GMC Sierra Code - Internal Mode Switch 'S' Circuit Low Voltage

Quick Answer

The P1840 diagnostic trouble code for a 2016 GMC Sierra indicates an issue with the Internal Mode Switch 'S' Circuit Low Voltage. Common causes include Faulty internal mode switch, Damaged wiring or connectors in the 'S' circuit, ECM malfunction. 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
P1840 2016 GMC Sierra indicates Internal Mode Switch 'S' Circuit Low Voltage.
Common causes
Common causes include Faulty internal mode switch, Damaged wiring or connectors in the 'S' circuit, ECM malfunction.
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.
What is this?
Table of contents Created and updated by William Orellana on Sep 14, 2024
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Code P1840 2016 GMC Sierra Description

The P1840 diagnostic trouble code for a 2016 GMC Sierra indicates an issue with the Internal Mode Switch 'S' Circuit Low Voltage. The internal mode switch (IMS) is a component within the transmission system that helps the vehicle's computer determine the current gear selection. The 'S' circuit refers to a specific circuit within the IMS that is reporting a low voltage signal to the vehicle's engine control module (ECM).

OBDII Code P1840 2016 GMC Sierra - Internal Mode Switch 'S' Circuit Low Voltage
P1840 2016 GMC Sierra Code - Internal Mode Switch 'S' Circuit Low Voltage

Common Causes of P1840 2016 GMC Sierra

  1. Faulty internal mode switch
  2. Damaged wiring or connectors in the 'S' circuit
  3. ECM malfunction
  4. Corrosion or poor connections in the circuit
  5. Electrical issues such as a blown fuse or short circuit

Symptoms of P1840 2016 GMC Sierra

  1. Erratic shifting or difficulty changing gears
  2. Harsh or delayed gear changes
  3. Transmission slipping
  4. Check Engine Light illuminated
  5. Reduced fuel efficiency

How to Fix P1840 2016 GMC Sierra

  1. Begin by conducting a thorough inspection of the internal mode switch, wiring, and connectors to identify any visible damage or issues.
  2. Test the internal mode switch using a multimeter to check for proper voltage output.
  3. Check the wiring harness and connectors for any signs of damage, corrosion, or poor connections. Repair or replace as needed.
  4. Clear the diagnostic trouble codes from the ECM and perform a test drive to ensure the issue has been resolved.
  5. If the problem persists, consider replacing the internal mode switch or consulting a professional technician for further diagnosis.

Cost to Fix P1840 2016 GMC Sierra

The typical repair costs for addressing the Internal Mode Switch 'S' Circuit Low Voltage issue can vary depending on the specific cause of the problem. In general, the cost of replacing the internal mode switch or repairing wiring/connectors can range from $200 to $500, including parts and labor. Additional diagnostic fees and labor rates at auto repair shops can impact the overall cost. It's recommended to consult with a trusted mechanic or dealership for a more accurate estimate based on your vehicle's make and model.

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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 P1840 2016 GMC Sierra Code

1. What are the common symptoms of OBDII code P1840 on a 2016 GMC Sierra?

- Common symptoms include issues with shifting gears, transmission slipping, difficulty engaging gears, and the vehicle may go into "limp mode."

2. What are the possible causes of the Internal Mode Switch 'S' Circuit Low Voltage code in a 2016 GMC Sierra?

- Potential causes include a faulty internal mode switch, damaged wiring or connectors, low voltage in the circuit, or a malfunctioning transmission control module.

3. How can I diagnose OBDII code P1840 on my 2016 GMC Sierra?

- Diagnosing this code typically involves using a scan tool to check for the specific fault code, testing the internal mode switch and related wiring, and inspecting connections for any damage.

4. Can driving with OBDII code P1840 on a 2016 GMC Sierra cause further damage?

- Continuing to drive with this code may lead to transmission issues and potentially cause further damage if not addressed promptly.

5. How can I repair OBDII code P1840 on my 2016 GMC Sierra?

- Repair options may include replacing the internal mode switch, repairing or replacing damaged wiring or connectors, or reprogramming the transmission control module.

6. Is it safe to continue driving my 2016 GMC Sierra with the Internal Mode Switch 'S' Circuit Low Voltage code?

- It is not recommended to drive the vehicle with this code as it can lead to transmission problems and safety hazards.

7. Can a DIY enthusiast fix OBDII code P1840 on a 2016 GMC Sierra?

- While some basic diagnostics can be performed by DIYers, fixing this code often requires professional tools and expertise due to the complexity of the transmission system.

8. How much does it cost to repair OBDII code P1840 on a 2016 GMC Sierra?

- The cost of repair can vary depending on the underlying issue, but it may involve parts and labor expenses ranging from a few hundred to over a thousand dollars.

9. Will resetting the OBDII code P1840 on a 2016 GMC Sierra make it go away permanently?

- Simply resetting the code without addressing the root cause will not fix the issue permanently. The problem should be diagnosed and repaired to prevent the code from reoccurring.

10. Can extreme weather conditions affect the occurrence of OBDII code P1840 on a 2016 GMC Sierra?

- Extreme weather conditions may exacerbate existing issues related to the internal mode switch or circuit, potentially triggering the code in a 2016 GMC Sierra.

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