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Engine-Codes – Generic – B3903

B3903 Code - Stop Lamp Relay Circuit Short

Quick Answer

The B3903 diagnostic trouble code (DTC) refers to a Stop Lamp Relay Circuit Short. Common causes include Faulty stop lamp relay, Short circuit in the wiring harness, Corrosion or loose connections in the 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 indicates Stop Lamp Relay Circuit Short.
Common causes
Common causes include Faulty stop lamp relay, Short circuit in the wiring harness, Corrosion or loose connections in the 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 Sep 08, 2024
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Code B3903 Description

The B3903 diagnostic trouble code (DTC) refers to a Stop Lamp Relay Circuit Short. This code indicates that there is a short circuit in the circuit that controls the stop lamp relay in the vehicle. The stop lamp relay is responsible for activating the brake lights when the brake pedal is pressed. When this circuit is shorted, it can cause issues with the brake lights, which are crucial for signaling to other drivers when the vehicle is slowing down or stopping.

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

Common Causes of B3903

  1. Faulty stop lamp relay
  2. Short circuit in the wiring harness
  3. Corrosion or loose connections in the circuit
  4. Faulty brake light switch
  5. Water damage to the wiring harness

Symptoms of B3903

  1. Brake lights not working
  2. Warning light on the dashboard
  3. Fuses related to the brake lights blowing frequently
  4. Brake lights staying on constantly

How to Fix B3903

  1. Begin by visually inspecting the wiring harness for any signs of damage, corrosion, or loose connections. Repair or replace any damaged wiring.
  2. Check the stop lamp relay for proper functionality. Replace the relay if it is found to be faulty.
  3. Test the brake light switch to ensure it is working correctly. Replace the switch if necessary.
  4. Clear the DTC from the vehicle's computer system using a diagnostic scan tool.
  5. Test the brake lights to ensure they are functioning properly after the repair.

Cost to Fix B3903

The cost of repairing a Stop Lamp Relay Circuit Short can vary depending on the specific cause of the issue and the labor rates of the auto repair shop. On average, the cost of parts for this repair could range from $50 to $150, while labor costs could range from $80 to $150 per hour. In total, the repair could cost anywhere from $150 to $300 or more. It is recommended to get a detailed estimate from a professional mechanic to get an accurate cost for the repair.

What does this mean?

Tech Notes for B3903

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

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Code B3903 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 B3903 Code

1. What does the OBDII code B3903 mean?

Code B3903 indicates a short circuit in the stop lamp relay circuit, which affects the functionality of the vehicle's brake lights.

2. What are the common symptoms of code B3903?

Symptoms include non-functional brake lights, illuminated warning lights on the dashboard, and possible failure of cruise control to engage.

3. What causes the B3903 code?

Common causes include a shorted or damaged stop lamp relay, faulty wiring or connectors in the brake light circuit, or a malfunction in the brake light switch.

4. Can I drive with a B3903 code?

It's not advisable as non-functional brake lights pose a serious safety risk to you and other drivers on the road.

5. How do I diagnose a B3903 code?

Diagnosis involves inspecting the stop lamp relay, checking wiring and connectors for damage, and using a multimeter to test for shorts in the circuit.

6. How can I fix the B3903 code?

Repairs may include replacing the stop lamp relay, fixing damaged wiring or connectors, or replacing a faulty brake light switch.

7. How much does it cost to repair a B3903 code?

Costs vary depending on the issue but typically range from $50 to $200, including parts and labor.

8. Will clearing the B3903 code fix the problem?

Clearing the code without addressing the underlying issue will likely result in the code returning and the problem persisting.

9. Can a faulty brake light switch trigger the B3903 code?

Yes, a defective brake light switch can cause this code due to its role in completing the circuit for the stop lamp relay.

10. How do I prevent the B3903 code from occurring?

Regularly inspect and maintain the brake light system, including the relay, wiring, and switch, to prevent issues that could lead to this code.

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