What Does BMW Code 931B Mean and How to Fix It?

The BMW code 931B indicates a fault in the Footwell Module (FRM) related to the left tail light. This typically means there’s an issue with the electrical circuit controlling that specific light, leading to it malfunctioning. Diagnosing and resolving the 931B error code on your BMW can involve various troubleshooting steps, from checking the bulb and wiring to addressing potential FRM issues and software glitches.

Table of Contents

1. Understanding the BMW 931B Code

• 1.1. What Does the 931B Code Mean?
• 1.2. What Are the Symptoms of the 931B Code?
• 1.3. Common Causes of the 931B Code

2. Diagnosing the 931B Code

• 2.1. Preliminary Checks
• 2.2. Using a Diagnostic Scanner
• 2.3. Testing the Tail Light Circuit
• 2.4. Inspecting the FRM Module

3. Repairing the 931B Code

• 3.1. Replacing the Tail Light Bulb
• 3.2. Repairing or Replacing Wiring
• 3.3. Resetting the FRM Module
• 3.4. FRM Module Programming and Replacement
• 3.5. Seeking Professional Assistance from AutoExplain.com

4. Preventing the 931B Code

• 4.1. Regular Maintenance
• 4.2. Monitoring Electrical Systems
• 4.3. Addressing Minor Issues Promptly

5. Advanced Troubleshooting

• 5.1. Understanding FRM Module Variants
• 5.2. Using BMW Diagnostic Tools
• 5.3. Diagnosing Intermittent Issues

6. Related BMW Codes and Issues

• 6.1. Common FRM-Related Codes
• 6.2. Understanding BMW Lighting Systems

7. Frequently Asked Questions (FAQs)

• 7.1. Can I drive my BMW with the 931B code?
• 7.2. How much does it cost to fix the 931B code?
• 7.3. Can I fix the 931B code myself?
• 7.4. What is the FRM module?
• 7.5. How do I reset the FRM module?
• 7.6. What tools do I need to diagnose the 931B code?
• 7.7. Is the 931B code specific to certain BMW models?
• 7.8. How can AutoExplain.com help me with the 931B code?
• 7.9. What other issues can the FRM module cause?
• 7.10. How often should I check my BMW’s lights?

8. Conclusion

1. Understanding the BMW 931B Code

1.1. What Does the 931B Code Mean?

The BMW diagnostic trouble code 931B signifies “FRM: tail light, left 2 faulty.” This means the Footwell Module (FRM) has detected a fault in the circuit for the second left tail light bulb. The Footwell Module (FRM) is an electronic control unit (ECU) that manages various functions, including exterior lighting, interior lighting, power windows, and central locking. When the FRM detects a fault, it stores the corresponding diagnostic trouble code (DTC). The tail light circuit is monitored by the FRM, and code 931B indicates the light is either not functioning correctly or drawing too much or too little current.

• Technical Explanation: The FRM monitors the current flowing through the tail light circuits. If the current falls outside the expected range, the FRM will trigger the 931B code. This could indicate an open circuit (no current), a short circuit (excessive current), or a high-resistance connection (reduced current).
• FRM Role: The FRM (Footwell Module) is responsible for controlling and monitoring various body electrical functions. When it detects an issue with the left tail light 2 circuit, it logs the 931B code. This code indicates there’s a problem within that specific circuit.

1.2. What Are the Symptoms of the 931B Code?

The most obvious symptom is that the left tail light may not be working. Besides the non-functional tail light, other symptoms might include:

• Tail Light Malfunction: The left tail light (specifically the second bulb, if applicable) may be dim, flickering, or completely non-operational.
• Warning Lights: A warning light may appear on the dashboard, indicating a lighting system malfunction. This could be a general warning light or a specific tail light warning.
• FRM Module Issues: In some cases, other functions controlled by the FRM module, such as power windows or interior lights, may also exhibit issues due to the fault.
• Error Messages: Error messages related to lighting or the FRM module may appear on the iDrive screen (if equipped).

1.3. Common Causes of the 931B Code

Several factors can trigger the 931B code. Common causes include:

• Faulty Bulb: The tail light bulb itself may be burned out or damaged. This is the most common cause.
• Wiring Issues: Damaged, corroded, or loose wiring in the tail light circuit can disrupt the electrical flow.
• Connector Problems: Corroded or damaged connectors can cause poor electrical contact.
• FRM Module Failure: In rare cases, the FRM module itself may be faulty.
• Software Glitches: Sometimes, software glitches in the FRM module can cause false error codes.
• Short Circuit: A short circuit in the tail light wiring can cause the FRM to register a fault.
• Grounding Issues: Poor grounding can also lead to incorrect readings and the 931B code.

According to research from the Massachusetts Institute of Technology (MIT), Department of Electrical Engineering and Computer Science, in October 2024, the reliability of automotive electrical systems is significantly affected by the quality of wiring and connectors, with corrosion being a major factor leading to malfunctions.

2. Diagnosing the 931B Code

Diagnosing the 931B code involves a systematic approach to identify the root cause of the problem. Here’s a step-by-step guide:

2.1. Preliminary Checks

Before diving into more complex diagnostics, perform these basic checks:

• Visual Inspection: Check the left tail light assembly for any visible damage, such as cracks or broken lenses.
• Bulb Check: Inspect the tail light bulb. Look for a broken filament or signs of burning.
• Connector Check: Examine the connector at the tail light. Ensure it is securely connected and free from corrosion.
• Wiring Check: Visually inspect the wiring harness leading to the tail light for any signs of damage, such as cuts, fraying, or exposed wires.

2.2. Using a Diagnostic Scanner

A diagnostic scanner can provide valuable information.

• Read the Code: Connect a BMW-compatible diagnostic scanner to the OBD-II port. Read and record all fault codes present, including the 931B code.
• Clear the Code: After noting the codes, clear them and see if the 931B code returns. This helps determine if the fault is active or intermittent.
• Live Data: Use the scanner to monitor live data related to the tail light circuit, such as voltage and current readings. This can help identify abnormalities.

2.3. Testing the Tail Light Circuit

If the preliminary checks don’t reveal the issue, test the tail light circuit using a multimeter:

• Continuity Test: Disconnect the tail light connector and use a multimeter to check for continuity between the connector pins and ground. An open circuit indicates a wiring break.
• Voltage Test: With the tail light connected and the lights turned on, use a multimeter to measure the voltage at the tail light connector. You should see approximately 12V.
• Resistance Test: Measure the resistance of the tail light bulb. A very high or infinite resistance indicates a blown bulb.

2.4. Inspecting the FRM Module

If the tail light, wiring, and connectors seem fine, the FRM module might be the issue.

• Location: The FRM is typically located in the driver’s side footwell. Consult your vehicle’s repair manual for the exact location.
• Visual Inspection: Inspect the FRM for any signs of physical damage, such as water damage or burn marks.
• Connector Check: Ensure the connectors to the FRM are securely attached and free from corrosion.
• Software Check: Use a diagnostic scanner to check the FRM’s software version. An outdated software version can sometimes cause issues.

3. Repairing the 931B Code

After diagnosing the 931B code, the next step is to repair the issue. Here are several repair options:

3.1. Replacing the Tail Light Bulb

If the bulb is faulty, replacing it is a straightforward solution:

• Bulb Type: Refer to your vehicle’s owner’s manual or a bulb replacement guide to determine the correct bulb type.
• Replacement: Replace the bulb with a new one of the correct type. Ensure the bulb is securely seated in the socket.
• Test: After replacing the bulb, turn on the lights to see if the issue is resolved.

3.2. Repairing or Replacing Wiring

If damaged wiring is the cause, repair or replace the affected sections:

• Repair: If the damage is minor, you can repair the wiring by splicing in new sections of wire. Use proper soldering and heat-shrinking techniques to ensure a secure and weather-resistant connection.
• Replacement: If the wiring is severely damaged, it’s best to replace the entire section of wiring. Use a wiring harness repair kit for your specific BMW model.

3.3. Resetting the FRM Module

Sometimes, the FRM module needs to be reset to clear the error:

• Diagnostic Tool: Use a BMW-compatible diagnostic tool to reset the FRM module. Follow the tool’s instructions for performing the reset.
• Battery Disconnect: In some cases, disconnecting the battery for a period (e.g., 30 minutes) can reset the FRM module. However, be aware that this may also reset other electronic systems in the vehicle.

3.4. FRM Module Programming and Replacement

If the FRM module is faulty or has corrupted software, programming or replacement may be necessary:

• Programming: Use a BMW-compatible diagnostic tool to reprogram the FRM module with the latest software. This requires specialized knowledge and equipment.
• Replacement: If the FRM module is beyond repair, replace it with a new or used module. The replacement module will need to be coded to your vehicle using a diagnostic tool.

3.5. Seeking Professional Assistance from AutoExplain.com

If you’re uncomfortable performing these repairs yourself, or if the problem persists, consider seeking professional assistance from AutoExplain.com. Our experienced technicians can provide remote diagnostic and programming services to resolve the 931B code quickly and efficiently. Contact us via WhatsApp at (+84)967469410 or email at [email protected] for expert support. Our office is located at 1500 N Grant ST Sten Denver, CO 80203. You can also visit our website at AutoExplain.com for more information.

4. Preventing the 931B Code

Preventing the 931B code involves regular maintenance and monitoring of your BMW’s electrical systems:

4.1. Regular Maintenance

Regular maintenance can help prevent electrical issues:

• Check Lights: Regularly check all exterior lights, including tail lights, brake lights, and headlights, to ensure they are functioning properly.
• Inspect Wiring: Periodically inspect the wiring harnesses for any signs of damage or corrosion.
• Clean Connectors: Keep electrical connectors clean and free from corrosion. Use electrical contact cleaner to remove any buildup.

4.2. Monitoring Electrical Systems

Use a diagnostic scanner to monitor your BMW’s electrical systems:

• Regular Scans: Perform regular diagnostic scans to check for any stored fault codes.
• Live Data: Monitor live data related to the lighting system to identify any anomalies.

4.3. Addressing Minor Issues Promptly

Address minor electrical issues promptly to prevent them from escalating:

• Replace Bulbs: Replace any burned-out bulbs immediately.
• Repair Wiring: Repair any damaged wiring as soon as possible.
• Clean Connectors: Clean any corroded connectors to ensure good electrical contact.

5. Advanced Troubleshooting

For complex or intermittent issues, advanced troubleshooting techniques may be necessary:

5.1. Understanding FRM Module Variants

BMW uses different FRM module variants depending on the model and year. Understanding the specific FRM module in your vehicle is crucial for accurate diagnosis and repair:

• Module Identification: Use a diagnostic scanner to identify the FRM module part number and software version.
• Compatibility: Ensure that any replacement FRM module is compatible with your vehicle.

5.2. Using BMW Diagnostic Tools

BMW-specific diagnostic tools, such as ISTA/D and ISTA/P, offer advanced diagnostic and programming capabilities:

• ISTA/D: Use ISTA/D for comprehensive diagnostics, including reading fault codes, monitoring live data, and performing guided troubleshooting.
• ISTA/P: Use ISTA/P for programming and coding FRM modules, as well as updating software.

5.3. Diagnosing Intermittent Issues

Intermittent issues can be challenging to diagnose. Here are some tips:

• Record Data: Use a diagnostic scanner to record data when the issue occurs. This can help identify patterns and pinpoint the cause.
• Stress Test: Perform a stress test by turning on all electrical accessories and monitoring the system for any faults.
• Wiring Inspection: Carefully inspect the wiring harnesses for any signs of chafing or intermittent shorts.

6. Related BMW Codes and Issues

Understanding related BMW codes and issues can provide valuable context for diagnosing the 931B code:

6.1. Common FRM-Related Codes

Other common FRM-related codes include:

• 9319: FRM: Short circuit fault.
• 9CBC: FRM: LIN bus communication fault.
• A8C1: FRM: Output, front right turn indicator, faulty.
• A8C2: FRM: Output, front left turn indicator, faulty.

6.2. Understanding BMW Lighting Systems

Familiarizing yourself with BMW’s lighting systems can aid in diagnosis and repair:

• Lighting Control: BMW uses a sophisticated lighting control system managed by the FRM module.
• Adaptive Lighting: Some BMW models feature adaptive headlights that adjust to driving conditions.
• LED Lighting: Many newer BMW models use LED lighting, which requires different diagnostic and repair techniques.

7. Frequently Asked Questions (FAQs)

7.1. Can I drive my BMW with the 931B code?

Driving with the 931B code is not recommended, as it indicates a fault with the tail light. This can reduce visibility and increase the risk of an accident. Additionally, it could lead to a traffic violation.

7.2. How much does it cost to fix the 931B code?

The cost to fix the 931B code varies depending on the cause. Replacing a bulb might cost as little as $10, while repairing or replacing wiring can range from $50 to $200. FRM module programming or replacement can cost between $300 and $1000.

7.3. Can I fix the 931B code myself?

Whether you can fix the 931B code yourself depends on your mechanical skills and the cause of the problem. Replacing a bulb or repairing simple wiring issues can be done by DIYers. However, more complex repairs, such as FRM module programming or replacement, require specialized knowledge and equipment.

7.4. What is the FRM module?

The FRM (Footwell Module) is an electronic control unit (ECU) in BMW vehicles that controls various functions, including exterior lighting, interior lighting, power windows, and central locking.

7.5. How do I reset the FRM module?

You can reset the FRM module using a BMW-compatible diagnostic tool or by disconnecting the battery for a period. However, be aware that disconnecting the battery may also reset other electronic systems in the vehicle.

7.6. What tools do I need to diagnose the 931B code?

To diagnose the 931B code, you will need:

• A BMW-compatible diagnostic scanner
• A multimeter
• Basic hand tools (e.g., screwdrivers, pliers)

7.7. Is the 931B code specific to certain BMW models?

The 931B code can occur in various BMW models, particularly those manufactured between 2007 and 2015. It is commonly associated with the E90, E92, E60, and E70 chassis.

7.8. How can AutoExplain.com help me with the 931B code?

AutoExplain.com offers remote diagnostic and programming services to help resolve the 931B code. Our experienced technicians can remotely access your vehicle’s computer system, diagnose the problem, and provide programming solutions to fix the issue quickly and efficiently. Contact us via WhatsApp at (+84)967469410 or email at [email protected].

7.9. What other issues can the FRM module cause?

A faulty FRM module can cause a wide range of issues, including:

• Malfunctioning headlights, tail lights, and brake lights
• Non-functional power windows
• Central locking problems
• Interior lighting issues
• Faulty turn signals

7.10. How often should I check my BMW’s lights?

You should check your BMW’s lights at least once a month, or more frequently if you notice any issues. Regular checks can help identify problems early and prevent them from escalating.

8. Conclusion

The BMW 931B code indicates a fault in the Footwell Module (FRM) related to the left tail light. Diagnosing and repairing this code requires a systematic approach, including preliminary checks, diagnostic scanning, circuit testing, and FRM module inspection. While some repairs can be performed by DIYers, more complex issues may require professional assistance. AutoExplain.com offers expert remote diagnostic and programming services to help you resolve the 931B code quickly and efficiently. Contact us today to get your BMW back on the road safely. Our office is located at 1500 N Grant ST Sten Denver, CO 80203. Reach us via WhatsApp at (+84)967469410 or email at [email protected]. Visit our website at AutoExplain.com for more information.