B1371 — Illuminated Entry Relay Circuit Failure

OBD-II trouble code B1371 indicates a problem with the relay circuit responsible for the vehicle’s illuminated entry system, commonly known as « welcome lights » or « fade-in lights ». This code suggests that the vehicle’s ECU (Engine Control Unit) has detected a malfunction in the circuit controlling the relay that powers these entry lights. It’s important to note that the exact terminology or label for this fault may vary depending on the vehicle manufacturer.

Manufacturer variations for trouble code B1371

Because diagnostic trouble codes (DTCs) are standardized through the OBD-II system, the B1371 code tends to have similar interpretations across brands, but labels or specific fault descriptions may vary. Different OEMs might assign unique identifiers or descriptions to this fault, especially in proprietary diagnostic systems.

• Volkswagen / Audi: « Comfort System – Relay Circuit Malfunction »
• Ford: « Interior Lighting Control Circuit Malfunction »
• BMW: « Lighting Module Relay Fault »
• Mercedes-Benz: « Body Control Module – Relay Fault »
• Toyota / Lexus: « Entry Light Relay Circuit Malfunction »

What does trouble code B1371 mean?

The B1371 code indicates that the vehicle’s Engine Control Unit (ECU), or more precisely the Body Control Module (BCM) in some models, has detected an abnormality in the relay circuit responsible for the illuminated entry system. This system often includes courtesy or welcome lights that turn on when unlocking the vehicle or opening a door. The ECU monitors the relay circuit to ensure these lights operate correctly. When a fault is identified—such as an open circuit, short, or communication problem—the system logs this code.

This code can be triggered by a variety of issues, including wiring problems, faulty relays, or a malfunctioning BCM. It usually appears during vehicle diagnostics when the system checks for circuit integrity or correct operation of the relays controlling the lighting features. Often, the problem is limited to an electronic circuit or relay that has failed or is experiencing intermittent contact issues.

Severity and risks of trouble code B1371

In most cases, the B1371 code is categorized as a moderate fault. It primarily affects convenience features, such as the illuminated or courtesy lights, enabling an improved vehicle entry experience. It does not usually interfere with essential vehicle functions like engine performance or safety systems.

However, if left unrepaired, the relay circuit fault may lead to the complete failure of the entry lighting system, which could be an inconvenience or even a safety concern if interior lights do not illuminate in low-light situations. Additionally, persistent electrical faults could, in rare instances, cause further damage to related components or wiring.

Overall, the vehicle can generally be driven with this fault present, but it is recommended to have it diagnosed and repaired promptly.

Symptoms of trouble code B1371

Typically, the presence of the B1371 fault code results in immediate observable issues regarding the vehicle’s lighting. Common symptoms include:

• Inoperative entry or courtesy lights: The interior or exterior welcoming lights do not turn on when unlocking or opening doors.
• Non-functioning relay: Audible click from relay or difficulty controlling the relay via vehicle switches.
• Dashboard warning lights: A warning indicator related to body control or lighting system may illuminate.
• Intermittent lighting behavior: Flickering or inconsistent operation of entry lights before failure.

Most likely causes of trouble code B1371

Understanding the causes behind the B1371 fault helps to streamline diagnosis and repair. While specific issues may vary between vehicles, the most common causes include:

1. Faulty relay: The relay responsible for illuminating entry or courtesy lights may be damaged or worn out, leading to circuit failure.
2. Wiring issues: Damaged, frayed, or corroded wiring or connectors in the relay circuit can interrupt proper function.
3. Malfunctioning Body Control Module (BCM): The BCM may have internal faults affecting relay control signals.
4. Blown fuse: A fuse dedicated to the lighting circuit might be blown, eliminating power flow.
5. Control switch failure: Switches associated with the lighting system might be malfunctioning or damaged.
6. Corrosion or water ingress: Exposure to moisture can impair relay contacts or wiring connections.

How to diagnose trouble code B1371

Diagnosing the B1371 code involves systematic checks to identify the root cause. Follow these generic steps to ensure a safe and effective test:

1. Visual inspection: Examine wiring harnesses, connectors, and relays related to the entry lighting circuit for corrosion, damage, or disconnection.
2. Check fuse integrity: Verify if the fuse linked to the lighting circuit is blown and replace if necessary.
3. Test relay operation: Swap the relay with a known good one or test it with a multimeter for proper contact closure.
4. Inspect switches: Confirm that door switches and interior lighting switches are functioning correctly and their wiring is intact.
5. Measure voltage and continuity: Use a multimeter to verify proper voltage supply and signal continuity along the wiring in the relay circuit.
6. Check for communication errors: Use a compatible OBD-II scan tool or vehicle-specific diagnostic software to review live data and BCM communication status.
7. Consult OEM diagrams: When necessary, refer to specific manufacturer wiring diagrams and repair procedures.

Possible repairs for trouble code B1371

Once the root causes are identified, repairs can be undertaken accordingly. The required actions vary from basic to professional repairs:

• Replace faulty relay: Simple replacement usually performed by a technician or knowledgeable vehicle owner.
• Repair wiring issues: Replacing or splicing wiring and connectors, best performed by a qualified professional to ensure safety and proper function.
• Replace blown fuse: A straightforward repair, often a DIY if the location is accessible.
• Reprogram or replace BCM: If the BCM is at fault, this requires specialized diagnostic tools and expertise, thus a professional repair is recommended.
• Replace defective switches: The door or interior light switches may need replacing, done by a technician.

Vehicles commonly associated with trouble code B1371 in Europe

This code has been reported across various vehicle brands, particularly those with advanced body control systems. While the specific prevalence may depend on model year and local conditions, common vehicle families include:

Volkswagen Group: Golf, Passat, Tiguan (various engine types)

Ford: Focus, Fiesta, Mondeo

BMW: 1 Series, 3 Series, X Models

Mercedes-Benz: C-Class, E-Class, SUVs

Toyota / Lexus: Corolla, Camry, RX, ES

Frequent mistakes with trouble code B1371

Diagnosing and repairing relay circuit faults can sometimes lead to errors. Common mistakes include:

• Replacing the relay without verifying circuit integrity: Changing the relay alone without inspecting wiring or BCM may not resolve the issue.
• Ignoring underlying wiring issues: Focusing only on the relay when the problem stems from corrosion, damage, or wiring shorts.
• Overlooking fuse checks: Not verifying the fuse’s condition can lead to unnecessary replacements or missed causes.
• Misinterpreting the code: Assuming it only affects interior lights; in some vehicles, the relay may control multiple systems.
• Skipping OEM diagnostic procedures: Relying solely on generic checks rather than manufacturer-specific diagnostics can result in incomplete repairs.

FAQ — trouble code B1371

Can this code disappear on its own? In most cases, no. The fault typically persists until the underlying issue is addressed. Temporary circuit anomalies, such as loose wiring, may cause intermittent codes but usually reoccur if not repaired.

Can I keep driving? Yes, generally the vehicle remains drivable since this is a convenience circuit related to lighting. However, if interior or exterior entry lights are crucial for visibility and safety, it’s advisable to diagnose and fix the issue promptly.

Why does the code return after clearing? The fault reappears if the original cause—such as a faulty relay, wiring short, or BCM issue—is not repaired. Recurrent faults point toward persistent hardware or connection problems.