AutoCodes – Buick – P1840 Buick

P1840 Buick 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 Buick 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.

Save to My Repairs My Repairs

AI answers can be wrong. Verify repairs with testing. Open full chat

Code Popularity 1,180

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
Add Comment

💬 Need more help? Ask Coddy for instant answers — it's free!

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

What does this mean?

What are the Possible Causes of the DTC P1840 Buick?

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 Buick?

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 Buick 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 Buick code?

Need more information on how to fix the P1840 Buick code? Get Access to Factory Service Manuals

Need Help with the Code?

Get the P1840 Buick code diagnosed by a professional: Find a repair shop in your area

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 Buick Code

1. What are the common symptoms of OBDII code P1840 in a Buick related to the Internal Mode Switch 'S' Circuit Low Voltage?

- Common symptoms include issues with shifting gears, hesitation during acceleration, and illuminated warning lights on the dashboard.

2. What are the possible causes of the Internal Mode Switch 'S' Circuit Low Voltage code P1840 in a Buick?

- The code can be triggered by a faulty internal mode switch, damaged wiring or connectors, low voltage in the circuit, or a problem with the transmission control module.

3. How can I diagnose the cause of OBDII code P1840 in my Buick?

- Diagnostic steps include checking the internal mode switch, inspecting wiring for damage, testing voltage in the circuit, and scanning the transmission control module for related faults.

4. Can driving with OBDII code P1840 present in a Buick cause further damage?

- Continuing to drive with this code may lead to transmission issues, poor vehicle performance, and potential safety hazards. It's recommended to address the problem promptly.

5. Are there any simple repairs I can try to fix OBDII code P1840 in my Buick?

- You may start by inspecting and cleaning the connectors, checking for loose wiring, and ensuring proper voltage levels. However, if the issue persists, professional diagnosis and repair may be necessary.

6. How much does it typically cost to repair OBDII code P1840 in a Buick?

- The cost of repair can vary depending on the exact cause of the code and the labor rates in your area. It's advisable to get multiple quotes from reputable mechanics or dealerships.

7. Can resetting the OBDII code P1840 in my Buick make it go away permanently?

- Resetting the code may temporarily turn off the warning light, but if the underlying issue is not fixed, the code is likely to reappear once the system detects the problem again.

8. Is it safe to drive my Buick with the Internal Mode Switch 'S' Circuit Low Voltage code P1840?

- Driving with this code can pose risks of transmission malfunctions and unpredictable vehicle behavior. It's recommended to address the issue promptly to ensure safe operation.

9. Does OBDII code P1840 in a Buick affect fuel efficiency?

- Yes, unresolved transmission issues related to this code can impact fuel efficiency due to improper shifting and engine performance. Resolving the problem may help improve gas mileage.

10. How long does it typically take to repair OBDII code P1840 in a Buick?

- The repair time can vary based on the complexity of the issue, availability of parts, and the proficiency of the mechanic. In some cases, simple fixes can be done quickly, while more involved repairs may take longer.