AutoCodes – GMC – P1840 2017 GMC Sierra

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

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: P1840 2017 GMC Sierra indicates Internal Mode Switch 'S' Circuit Low Voltage.
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 186

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 P1840 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 S signal circuit voltage is less than 0.7 V, for 1.75 s.

OBDII Code P1840 2017 GMC Sierra - Internal Mode Switch 'S' Circuit Low Voltage - AutoCodes.com — P1840 2017 GMC Sierra Code - Internal Mode Switch 'S' Circuit Low Voltage

What does this mean?

What are the Possible Causes of the DTC P1840 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 P1840 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 P1840 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 P1840 2017 GMC Sierra 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 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.