AutoCodes – GMC – B3903 GMC

B3903 GMC Code - Stop Lamp Relay Circuit Short

Quick Answer

The Body Control Module (BCM) supplies voltage to the Center High Mount Stop Lamp (CHMSL) and the Powertrain Control Module (PCM) when the stop lamps are active. Common causes include Faulty Stop Lamp Relay, Stop Lamp Relay harness is open or shorted, Stop Lamp Relay 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: B3903 GMC indicates Stop Lamp Relay Circuit Short.
Common causes: Common causes include Faulty Stop Lamp Relay, Stop Lamp Relay harness is open or shorted, Stop Lamp Relay 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 572

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 25, 2020

Description
Possible Causes
How To Fix
Tech Notes
Cost to Diagnose
FAQs
Comments
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Code B3903 GMC Description

The Body Control Module (BCM) supplies voltage to the Center High Mount Stop Lamp (CHMSL) and the Powertrain Control Module (PCM) when the stop lamps are active. The Diagnostic Trouble Code (DTC) sets when if the stop lamp switch signal circuit is shorted to ground for 125 ms or if is open or shorted to voltage for 125 ms.

OBDII Code B3903 GMC - Stop Lamp Relay Circuit Short - AutoCodes.com — B3903 GMC Code - Stop Lamp Relay Circuit Short

What does this mean?

What are the Possible Causes of the DTC B3903 GMC?

Faulty Stop Lamp Relay
Stop Lamp Relay harness is open or shorted
Stop Lamp Relay circuit poor electrical connection
Faulty Body Control Module (BCM)

How to Fix the DTC B3903 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 does this mean?

Code Tech Notes

B3903 02 Stop Lamp Relay Circuit Short to Ground
B3903 05 Stop Lamp Relay Circuit Short to Battery or Open

What is the Cost to Diagnose the Code?

Labor: 1.0

To diagnose the B3903 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.

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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 B3903 GMC Code

1. What are the common symptoms of OBDII code B3903 GMC (Stop Lamp Relay Circuit Short)?

The common symptoms include non-functioning brake lights, illuminated brake warning light on the dashboard, and possibly issues with other electrical components related to the brake system.

2. What are the possible causes of OBDII code B3903 GMC (Stop Lamp Relay Circuit Short)?

The code may be triggered by a malfunctioning stop lamp relay, damaged wiring or connectors in the stop lamp circuit, or a short circuit in the brake light system.

3. How can I diagnose the cause of OBDII code B3903 GMC (Stop Lamp Relay Circuit Short)?

You can start by visually inspecting the stop lamp relay, checking for damaged wiring or connectors, and using a multimeter to test for continuity and shorts in the circuit.

4. Can I still drive my vehicle with OBDII code B3903 GMC (Stop Lamp Relay Circuit Short)?

It is not recommended to drive your vehicle with non-functioning brake lights, as it poses a safety risk. Have the issue diagnosed and repaired as soon as possible.

5. How do I repair OBDII code B3903 GMC (Stop Lamp Relay Circuit Short)?

Repair methods may include replacing the faulty stop lamp relay, repairing or replacing damaged wiring or connectors, and clearing the trouble code with an OBDII scanner.

6. Will resetting the OBDII code B3903 GMC (Stop Lamp Relay Circuit Short) make the issue go away?

Resetting the code may temporarily turn off the warning light, but if the underlying issue causing the code is not addressed, it is likely to reoccur.

7. Can a DIY enthusiast fix OBDII code B3903 GMC (Stop Lamp Relay Circuit Short)?

Depending on your skill level and access to tools, a DIY enthusiast may be able to diagnose and repair the issue. However, electrical issues can be tricky, so professional help may be needed.

8. Are there any safety precautions to take when working on OBDII code B3903 GMC (Stop Lamp Relay Circuit Short)?

When working on the brake light system, always disconnect the battery, wear safety goggles, and follow proper procedures to avoid electrical shocks or short circuits.

9. How much does it cost to repair OBDII code B3903 GMC (Stop Lamp Relay Circuit Short)?

The cost of repair can vary depending on the underlying issue, labor rates, and parts needed. It is best to obtain quotes from different mechanics or repair shops.

10. Can OBDII code B3903 GMC (Stop Lamp Relay Circuit Short) affect the overall performance of the vehicle?

While the code itself may not affect the performance of the vehicle, non-functioning brake lights can compromise safety on the road. It is crucial to address the issue promptly.