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Engine-Codes – GMC – B3903 GMC

B3903 GMC Code - Stop Lamp Relay Circuit Short

Quick Answer

The B3903 code indicates a fault in the stop lamp relay circuit, specifically a short circuit condition. Common causes include A short circuit in the wiring harness leading to the stop lamp relay, A malfunctioning stop lamp relay that is stuck in the closed position, Damaged or corroded connectors within the brake light circuit. 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 A short circuit in the wiring harness leading to the stop lamp relay, A malfunctioning stop lamp relay that is stuck in the closed position, Damaged or corroded connectors within the brake light circuit.
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 Oct 17, 2024
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Code B3903 GMC Description

The B3903 code indicates a fault in the stop lamp relay circuit, specifically a short circuit condition. This code is prevalent in GMC vehicles and is associated with the vehicle's brake light system. The stop lamp relay is responsible for controlling the power supply to the brake lights, ensuring that they illuminate when the brake pedal is pressed. When a short circuit occurs in this circuit, it can lead to various performance issues, including the brake lights remaining illuminated at all times or failing to illuminate when the brake pedal is engaged.

OBDII Code B3903 GMC - Stop Lamp Relay Circuit Short
B3903 GMC Code - Stop Lamp Relay Circuit Short

Common Causes of B3903 GMC

  1. A short circuit in the wiring harness leading to the stop lamp relay.
  2. A malfunctioning stop lamp relay that is stuck in the closed position.
  3. Damaged or corroded connectors within the brake light circuit.
  4. Frayed or exposed wiring due to wear and tear or environmental factors.
  5. Faulty brake light switch that may be sending incorrect signals to the relay.

Symptoms of B3903 GMC

  1. Brake lights remain illuminated even when the brake pedal is not pressed.
  2. Brake lights do not illuminate when the brake pedal is engaged.
  3. Dashboard warning lights may illuminate, indicating a fault in the brake system.
  4. Possible blown fuses related to the brake light circuit.
  5. Intermittent electrical issues, such as flickering lights or erratic behavior of other electrical components.

How to Fix B3903 GMC

  1. Visual Inspection: Start with a thorough visual inspection of the wiring harness, connectors, and relay for any signs of damage, corrosion, or shorts.
  2. Testing the Relay: Use a multimeter to test the stop lamp relay for continuity and proper operation. Replace the relay if it is found to be faulty.
  3. Wiring Repair: If damaged wiring is identified, repair or replace the affected sections to ensure proper electrical flow.
  4. Brake Light Switch Check: Inspect the brake light switch for proper operation and replace it if it is malfunctioning.
  5. Clear Codes and Test: After repairs are made, clear the diagnostic trouble codes using an OBD-II scanner and perform a test drive to ensure the issue is resolved and the brake lights function correctly.

Cost to Fix B3903 GMC

The typical repair costs for addressing a B3903 code can vary widely based on the specific issue, parts needed, and labor rates in your area. On average, you might expect to pay between $100 and $300 for repairs, which includes parts and labor. If the issue is straightforward, such as replacing a relay or switch, costs may be on the lower end. However, if extensive wiring repairs are needed, costs can increase significantly.

What does this mean?

Tech Notes for B3903 GMC

B3903 02 Stop Lamp Relay Circuit Short to Ground

B3903 05 Stop Lamp Relay Circuit Short to Battery or Open

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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.

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