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P1840 2017 GMC Sierra Code - Internal Mode Switch 'S' Circuit Low Voltage

Quick Answer

The P1840 diagnostic trouble code (DTC) for a 2017 GMC Sierra refers to the Internal Mode Switch 'S' Circuit Low Voltage. Common causes include Faulty internal mode switch, Damaged wiring or connectors, Malfunctioning transmission control module (TCM). 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 2017 GMC Sierra indicates Internal Mode Switch 'S' Circuit Low Voltage.
Common causes
Common causes include Faulty internal mode switch, Damaged wiring or connectors, Malfunctioning transmission control module (TCM).
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 15, 2024
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Code P1840 2017 GMC Sierra Description

The P1840 diagnostic trouble code (DTC) for a 2017 GMC Sierra refers to the Internal Mode Switch 'S' Circuit Low Voltage. This code indicates that there is an issue with the internal mode switch, specifically related to low voltage. The internal mode switch is a component of the transmission system that helps the vehicle's computer determine the desired gear position based on driver input. When the internal mode switch 'S' circuit is experiencing low voltage, it can lead to incorrect gear selection, erratic shifting, and potential transmission issues.

OBDII Code P1840 2017 GMC Sierra - Internal Mode Switch 'S' Circuit Low Voltage
P1840 2017 GMC Sierra Code - Internal Mode Switch 'S' Circuit Low Voltage

Common Causes of P1840 2017 GMC Sierra

  1. Faulty internal mode switch
  2. Damaged wiring or connectors
  3. Malfunctioning transmission control module (TCM)
  4. Electrical issues such as a weak battery or poor ground connections
  5. Water or corrosion in the transmission system

Symptoms of P1840 2017 GMC Sierra

  1. Erratic shifting or difficulty changing gears
  2. Transmission slipping or hesitation
  3. Check Engine Light illuminated
  4. Reduced fuel efficiency
  5. Transmission stuck in a single gear

How to Fix P1840 2017 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 and related components using a multimeter to check for proper voltage and continuity.
  3. If the internal mode switch is found to be faulty, replace it with a new, OEM-quality component.
  4. Check and repair any damaged wiring or connectors in the circuit to ensure proper electrical connections.
  5. Clear the DTC from the vehicle's computer using a diagnostic scan tool and test drive the vehicle to verify that the issue has been resolved.

Cost to Fix P1840 2017 GMC Sierra

The typical repair costs for addressing the P1840 Internal Mode Switch 'S' Circuit Low Voltage code can vary depending on the specific cause of the issue and the labor rates in your area. On average, the cost of parts for this repair could range from $100 to $300, while labor costs could range from $150 to $400. It's important to note that these are rough estimates, and actual costs may vary based on factors such as the location of the repair shop, the make and model of the vehicle, and the extent of the repairs needed. It's recommended to obtain quotes from multiple repair shops to compare prices and ensure you are getting a fair deal.

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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 2017 GMC Sierra Code

1. What are the common symptoms of OBDII code P1840 in a 2017 GMC Sierra?

- Common symptoms include issues with shifting gears, hesitation or jerking during acceleration, and the inability to shift into certain gears.

2. What can cause the Internal Mode Switch 'S' Circuit Low Voltage error in a 2017 GMC Sierra?

- The error code P1840 can be triggered by a faulty internal mode switch, damaged wiring or connectors, or a malfunctioning transmission control module.

3. How can I diagnose the Internal Mode Switch 'S' Circuit Low Voltage issue in my GMC Sierra?

- Diagnosing the problem involves checking the internal mode switch for proper operation, inspecting the wiring and connectors for damage, and testing the transmission control module.

4. Can a DIY enthusiast repair the P1840 error code in a 2017 GMC Sierra?

- While some DIY enthusiasts may be able to address the issue by replacing the internal mode switch or repairing wiring, it is recommended to consult a professional mechanic for accurate diagnosis and repair.

5. Are there any quick fixes or temporary solutions for the P1840 code in a GMC Sierra?

- Quick fixes like resetting the error code using an OBDII scanner may temporarily clear the error, but it is crucial to address the underlying issue to prevent further transmission problems.

6. What are the potential risks of ignoring the Internal Mode Switch 'S' Circuit Low Voltage error in a GMC Sierra?

- Ignoring the P1840 error code can lead to more severe transmission issues, including transmission failure, which can result in costly repairs and safety risks on the road.

7. How much does it typically cost to repair the P1840 error in a 2017 GMC Sierra?

- The cost of repairing the Internal Mode Switch 'S' Circuit Low Voltage issue can vary depending on the root cause of the problem, but it may range from a few hundred to over a thousand dollars.

8. Is it safe to continue driving a 2017 GMC Sierra with the P1840 code present?

- It is not recommended to continue driving the vehicle with the P1840 error code as it can lead to unpredictable transmission behavior and potential safety hazards on the road.

9. Can weather conditions or external factors trigger the Internal Mode Switch 'S' Circuit Low Voltage error in a GMC Sierra?

- While extreme weather conditions or water exposure can potentially affect the internal mode switch or wiring, the error code is more commonly caused by internal component failures or electrical issues.

10. How long does it typically take to repair the P1840 error in a 2017 GMC Sierra?

- The repair time for the Internal Mode Switch 'S' Circuit Low Voltage issue can vary depending on the extent of the problem and the availability of parts, but it may take a few hours to a day for a professional mechanic to complete the repair.

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