AutoCodes – Cadillac – P1840 Cadillac

P1840 Cadillac 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 Cadillac 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 1,426

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

What does this mean?

What are the Possible Causes of the DTC P1840 Cadillac?

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 Cadillac?

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 Cadillac 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 Cadillac code?

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

Need Help with the Code?

Get the P1840 Cadillac 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 Cadillac Code

1. What are the common symptoms of OBDII code P1840 in a Cadillac?

The common symptoms of OBDII code P1840 include issues with shifting gears, difficulty starting the vehicle, and potential stalling.

2. What can cause the Internal Mode Switch 'S' Circuit Low Voltage error code in a Cadillac?

The error code P1840 can be caused by a faulty internal mode switch, damaged wiring or connectors, a malfunctioning transmission control module (TCM), or a low voltage condition.

3. How do I diagnose the Internal Mode Switch 'S' Circuit Low Voltage issue in my Cadillac?

To diagnose the P1840 code, you can start by checking the wiring and connectors for any visible damage, testing the internal mode switch for proper operation, and using a scan tool to check for TCM-related issues.

4. Can I continue driving my Cadillac with the P1840 error code present?

It is generally not recommended to continue driving your Cadillac with the P1840 code, as it can lead to further transmission problems and potential safety issues. It is best to address the underlying cause promptly.

5. What are some repair tips for fixing the Internal Mode Switch 'S' Circuit Low Voltage code in a Cadillac?

Repair tips for the P1840 code may include repairing or replacing damaged wiring, connectors, or the internal mode switch, resetting the TCM, and clearing the error code with a scan tool after the repairs are completed.

6. Is it possible to reset the OBDII code P1840 in my Cadillac without fixing the underlying issue?

While you can reset the error code temporarily, it is essential to fix the underlying issue causing the P1840 code to prevent it from reoccurring and potentially causing more significant problems in the future.

7. Does the Internal Mode Switch 'S' Circuit Low Voltage code affect the overall performance of my Cadillac?

Yes, the P1840 code can affect the overall performance of your Cadillac by causing transmission-related issues, impacting gear shifting, and potentially leading to drivability problems.

8. How much does it typically cost to repair the P1840 error code in a Cadillac?

The cost of repairing the P1840 code in a Cadillac can vary depending on the underlying cause, parts needed for replacement, labor costs, and whether you choose to do the repairs yourself or take your vehicle to a professional mechanic.

9. Can a DIY enthusiast fix the Internal Mode Switch 'S' Circuit Low Voltage issue in a Cadillac?

Depending on your knowledge and experience with automotive repairs, a DIY enthusiast may be able to fix the P1840 code by following diagnostic procedures, checking and repairing wiring or connectors, and replacing components if needed.

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

The repair time for the P1840 code can vary based on the complexity of the issue, availability of parts, and whether you are doing the repairs yourself or taking your vehicle to a professional technician. It may range from a few hours to a day or more.