AutoCodes – GMC – P1840 2016 GMC Sierra

P1840 2016 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 2016 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 162

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

OBDII Code P1840 2016 GMC Sierra - Internal Mode Switch 'S' Circuit Low Voltage - AutoCodes.com — P1840 2016 GMC Sierra Code - Internal Mode Switch 'S' Circuit Low Voltage

What does this mean?

What are the Possible Causes of the DTC P1840 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 P1840 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 P1840 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 P1840 2016 GMC Sierra code?

Need more information on how to fix the P1840 2016 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 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.