AutoCodes – GMC – P1840 2018 GMC Sierra

P1840 2018 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 2018 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 195

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 2018 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 2018 GMC Sierra - Internal Mode Switch 'S' Circuit Low Voltage - AutoCodes.com — P1840 2018 GMC Sierra Code - Internal Mode Switch 'S' Circuit Low Voltage

What does this mean?

What are the Possible Causes of the DTC P1840 2018 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 2018 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 2018 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 2018 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 2018 GMC Sierra Code

1. What are common symptoms of the OBDII code P1840 in a 2018 GMC Sierra?

- Common symptoms include issues with gear shifting, the transmission staying in one gear, and warning lights on the dashboard.

2. What does the Internal Mode Switch 'S' Circuit Low Voltage mean in relation to P1840 in a 2018 GMC Sierra?

- This code indicates a problem with the internal mode switch in the transmission control module, specifically a low voltage issue.

3. What are possible causes of the Internal Mode Switch 'S' Circuit Low Voltage code in a 2018 GMC Sierra?

- Potential causes include a faulty internal mode switch, damaged wiring or connectors, low battery voltage, or issues with the transmission control module.

4. How can I diagnose the Internal Mode Switch 'S' Circuit Low Voltage issue in my 2018 GMC Sierra?

- Diagnosis typically involves checking the internal mode switch for proper operation, inspecting wiring and connectors for damage, and testing the voltage levels in the circuit.

5. Can I drive my 2018 GMC Sierra with the P1840 code present?

- It is not recommended to drive the vehicle with this code present, as it can lead to transmission issues and potentially cause further damage.

6. How can I repair the Internal Mode Switch 'S' Circuit Low Voltage problem in my 2018 GMC Sierra?

- Repair may involve replacing the internal mode switch, repairing any damaged wiring or connectors, or resetting the transmission control module.

7. Will resetting the OBDII code P1840 in my 2018 GMC Sierra make it go away permanently?

- Resetting the code may temporarily clear the warning light, but if the underlying issue is not addressed, the code is likely to reappear.

8. How much does it cost to repair the P1840 code in a 2018 GMC Sierra at a dealership or mechanic?

- The cost of repair can vary depending on the exact cause of the issue, but it is recommended to get a diagnostic at a dealership or mechanic for an accurate estimate.

9. Are there any DIY tips for troubleshooting the P1840 code in a 2018 GMC Sierra?

- While basic checks like inspecting wiring and connectors can be done by DIY enthusiasts, diagnosing and repairing the internal mode switch issue may require professional expertise.

10. Is the Internal Mode Switch 'S' Circuit Low Voltage problem a common issue in 2018 GMC Sierra vehicles?

- While not extremely common, this issue has been reported by some GMC Sierra owners and should be addressed promptly to prevent further transmission problems.