AutoCodes – GMC – P1839 GMC

P1839 GMC Code - Internal Mode Switch 'C' 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: P1839 GMC indicates Internal Mode Switch 'C' 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 2,327

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 P1839 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 C signal circuit voltage is less than 0.7 V, for 1.75 s.

OBDII Code P1839 GMC - Internal Mode Switch 'C' Circuit Low Voltage - AutoCodes.com — P1839 GMC Code - Internal Mode Switch 'C' Circuit Low Voltage

What does this mean?

What are the Possible Causes of the DTC P1839 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 P1839 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 P1839 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 P1839 GMC code?

Need more information on how to fix the P1839 GMC code? Get Access to Factory Service Manuals

Need Help with the Code?

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Information for Specific GMC Models

P1839 2014 GMC Sierra — Internal Mode Switch 'C' Circuit Low Voltage
P1839 2015 GMC Sierra — Internal Mode Switch 'C' Circuit Low Voltage
P1839 2016 GMC Sierra — Internal Mode Switch 'C' Circuit Low Voltage
P1839 2017 GMC Sierra — Internal Mode Switch 'C' Circuit Low Voltage
P1839 2018 GMC Sierra — Internal Mode Switch 'C' 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 P1839 GMC Code

1. What are the common symptoms of OBDII code P1839 GMC (Internal Mode Switch 'C' Circuit Low Voltage)?

Common symptoms include issues with shifting gears, erratic transmission behavior, the inability to shift out of a particular gear, and warning lights on the dashboard.

2. What can cause the OBDII code P1839 GMC to trigger?

The code P1839 GMC is typically triggered by a malfunction in the internal mode switch 'C' circuit, which can be caused by wiring issues, a faulty internal mode switch, or a problem with the transmission control module.

3. How serious is the OBDII code P1839 GMC (Internal Mode Switch 'C' Circuit Low Voltage)?

While the code itself may not cause immediate drivability issues, ignoring it can lead to transmission problems, inefficient fuel consumption, and potential safety hazards. It's best to address the issue promptly.

4. How can I diagnose the cause of OBDII code P1839 GMC?

Diagnosing the code usually involves inspecting the wiring related to the internal mode switch 'C' circuit, testing the internal mode switch for proper functionality, and checking the transmission control module for any faults using a diagnostic scanner.

5. Can I repair the OBDII code P1839 GMC issue myself?

If you have experience working on automotive electrical systems and diagnostics, you may be able to troubleshoot and repair the issue yourself. However, if you are unsure or don't have the necessary tools, it's best to seek professional help.

6. Will driving with the OBDII code P1839 GMC illuminated cause further damage?

Continuing to drive with the code triggered may exacerbate transmission problems over time, leading to more extensive repairs and potentially higher costs. It's advisable to address the issue promptly to prevent further damage.

7. How much does it typically cost to repair OBDII code P1839 GMC?

The cost of repairing the code can vary depending on the underlying cause, parts needed for replacement, and labor rates in your area. It's best to consult with a mechanic or dealership for an accurate estimate.

8. Can resetting the OBDII code P1839 GMC make it go away permanently?

Simply resetting the code without addressing the underlying issue will not make it disappear permanently. The problem causing the code will need to be diagnosed and repaired to prevent its recurrence.

9. Is it safe to continue driving my vehicle with the OBDII code P1839 GMC pending?

While you may be able to drive with the code pending, it's important to address the issue as soon as possible to avoid potential transmission issues and ensure the safety and reliability of your vehicle.

10. How long does it take to repair OBDII code P1839 GMC?

The repair time for the code can vary depending on the specific cause and the complexity of the repair. It could range from a quick fix involving a wiring repair to a more involved process if component replacement is necessary.