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Engine-Codes – Generic – P1840

P1840 Code - Internal Mode Switch 'S' Circuit Low Voltage

Quick Answer

The P1840 diagnostic trouble code (DTC) indicates an issue with the Internal Mode Switch 'S' Circuit, specifically that the voltage is too low. Common causes include Faulty Internal Mode Switch (IMS), Corroded or damaged wiring/connectors in the 'S' circuit, Issues with the transmission control module (TCM). 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 indicates Internal Mode Switch 'S' Circuit Low Voltage.
Common causes
Common causes include Faulty Internal Mode Switch (IMS), Corroded or damaged wiring/connectors in the 'S' circuit, Issues with the transmission control module (TCM).
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.
What is this?
Table of contents Created and updated by William Orellana on Sep 06, 2024
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Code P1840 Description

The P1840 diagnostic trouble code (DTC) indicates an issue with the Internal Mode Switch 'S' Circuit, specifically that the voltage is too low. The Internal Mode Switch (IMS) is a crucial component of the transmission control system, responsible for providing input to the transmission control module (TCM) about the gear selection. When the voltage in the 'S' circuit is too low, it can lead to incorrect gear selection, erratic shifting, and potential transmission issues. This can ultimately affect the vehicle's performance, fuel efficiency, and overall drivability.

OBDII Code P1840 - Internal Mode Switch 'S' Circuit Low Voltage
P1840 Code - Internal Mode Switch 'S' Circuit Low Voltage

Common Causes of P1840

  1. Faulty Internal Mode Switch (IMS).
  2. Corroded or damaged wiring/connectors in the 'S' circuit.
  3. Issues with the transmission control module (TCM).
  4. Low battery voltage affecting the circuit.
  5. Water or debris entering the IMS causing malfunction.

Symptoms of P1840

  1. Incorrect gear selection displayed on the instrument cluster.
  2. Harsh or delayed shifting between gears.
  3. Transmission slipping or hesitation during acceleration.
  4. Engine stalling when shifting gears.
  5. Illumination of the Check Engine Light.

How to Fix P1840

  1. Begin by conducting a thorough inspection of the Internal Mode Switch and the 'S' circuit wiring for any visible damage or corrosion.
  2. Test the Internal Mode Switch using a multimeter to check for proper voltage readings.
  3. Replace the Internal Mode Switch if it is found to be faulty.
  4. Repair or replace any damaged wiring or connectors in the 'S' circuit.
  5. Clear the DTC codes and perform a test drive to ensure the issue has been resolved.

Cost to Fix P1840

The cost of repairing the Internal Mode Switch 'S' Circuit Low Voltage issue can vary depending on the specific cause of the problem. In general, the cost of a new Internal Mode Switch can range from $50 to $150, while the labor cost for diagnosis and repair can be between $100 to $300. Overall, the total repair cost could be in the range of $150 to $450. It's recommended to consult with a professional mechanic or auto repair shop for a more accurate cost estimate based on your vehicle's make and model.

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Code P1840 Information for Specific Makes

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 Code

1. What are common symptoms of OBDII code P1840?

Common symptoms include issues with shifting gears, difficulty engaging the transmission, and potentially a "Check Engine" light illuminated on the dashboard.

2. What causes the Internal Mode Switch 'S' Circuit Low Voltage code (P1840) to trigger?

The code P1840 is typically triggered by a malfunction in the internal mode switch (IMS) circuit, often due to wiring issues, a faulty IMS, or a problem with the transmission control module.

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

You can start by checking the wiring connected to the internal mode switch for any damage or loose connections. Using a multimeter to test the voltage at the switch can also help pinpoint the issue.

4. Can a DIY enthusiast repair OBDII code P1840, or is it best left to professionals?

Depending on your skill level and comfort working with automotive electronics, repairing P1840 may be feasible for some DIY enthusiasts. However, if you are unsure or lack experience, it's recommended to seek help from a professional mechanic.

5. Is it safe to continue driving with OBDII code P1840 present?

It is not recommended to continue driving with the P1840 code active, as it can lead to further transmission issues and potential safety hazards. It's best to address the problem promptly.

6. What are some potential repair options for resolving OBDII code P1840?

Repair options may include fixing or replacing the internal mode switch, repairing any damaged wiring, resetting the code after making necessary repairs, and ensuring the transmission control module is functioning correctly.

7. How much can I expect to pay for repairs related to OBDII code P1840?

The cost of repairs for code P1840 can vary depending on the underlying issue, parts needed, and labor costs. It's advisable to get estimates from different mechanics or repair shops to compare prices.

8. Can a low vehicle battery voltage cause OBDII code P1840 to trigger?

Yes, a low vehicle battery voltage can potentially cause the internal mode switch 'S' circuit to have low voltage, triggering code P1840. Ensuring the battery is in good condition is essential for proper electrical system operation.

9. Are there any preventive maintenance steps to avoid OBDII code P1840 in the future?

Regularly inspecting the internal mode switch, checking for wiring issues, and maintaining the transmission system can help prevent the recurrence of code P1840. Keeping up with scheduled maintenance can also help catch potential problems early.

10. If I clear the OBDII code P1840, how long should I wait to see if it reoccurs?

After clearing the P1840 code, it's recommended to drive the vehicle for a few days under normal conditions to see if the code reappears. If the issue persists, further diagnostics and repairs may be necessary.

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