AutoCodes – GMC – P1840 GMC

P1840 GMC 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 GMC 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 1,966

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

What does this mean?

What are the Possible Causes of the DTC P1840 GMC?

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 GMC?

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 GMC 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 GMC code?

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Need Help with the Code?

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Information for Specific GMC Models

P1840 2014 GMC Sierra — Internal Mode Switch 'S' Circuit Low Voltage
P1840 2015 GMC Sierra — Internal Mode Switch 'S' Circuit Low Voltage
P1840 2016 GMC Sierra — Internal Mode Switch 'S' Circuit Low Voltage
P1840 2017 GMC Sierra — Internal Mode Switch 'S' Circuit Low Voltage
P1840 2018 GMC Sierra — Internal Mode Switch 'S' Circuit Low Voltage

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 GMC Code

1. What are common symptoms of OBDII code P1840 GMC (Internal Mode Switch 'S' Circuit Low Voltage)?

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

2. What are the possible causes of OBDII code P1840 GMC?

- The code may be triggered due to a faulty internal mode switch, damaged wiring or connectors, a defective transmission range sensor, or a problem with the transmission control module.

3. How can I diagnose the cause of OBDII code P1840 GMC?

- Diagnostic steps may include checking the internal mode switch for proper operation, inspecting wiring and connectors for damage, testing the transmission range sensor, and scanning the transmission control module for other related error codes.

4. Can I continue driving my vehicle with OBDII code P1840 GMC?

- It is not recommended to continue driving the vehicle with this code present, as it can lead to further transmission issues and potential safety hazards.

5. How can I repair OBDII code P1840 GMC?

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

6. Is it possible to clear OBDII code P1840 GMC without fixing the underlying issue?

- Clearing the code without addressing the root cause will likely result in the code reappearing once the issue is detected again during vehicle operation.

7. Are there any DIY fixes for OBDII code P1840 GMC?

- DIY fixes may be limited due to the complexity of the transmission system, but basic checks like inspecting wiring connections and ensuring the internal mode switch is functioning properly can be done by experienced DIYers.

8. How much does it typically cost to repair OBDII code P1840 GMC at a repair shop?

- Repair costs can vary depending on the specific cause of the code, but expect to pay anywhere from $200 to $500 for parts and labor at a repair shop.

9. Can a mechanic reset OBDII code P1840 GMC without making any repairs?

- While a mechanic can clear the code from the system, it is important to address the underlying issue to prevent further problems with the transmission.

10. How long does it take to repair OBDII code P1840 GMC?

- The repair time can vary based on the exact cause of the code and the availability of parts, but most repairs related to this code can typically be completed within a few hours to a day at a repair shop.