Diagnosing & Fixing B12E8 & B121C Fault Codes in Audi Q8

Introduction

Imagine this: You’re driving your Audi Q8 at night, and suddenly, a warning message pops up about your daytime running lights (DRLs) or headlight system. Maybe your DRLs are flickering, or your headlights dim unexpectedly. A quick diagnostic scan reveals fault codes B12E8 or B121C—what do they mean, and how do you fix them?

At AUTOTEK, we help car owners and mechanics make informed repair decisions. This guide will walk you through diagnosing, troubleshooting, and fixing these issues in your Audi Q8.

Understanding B12E8 & B121C Fault Codes

Fault Code B12E8 – Daytime Running Light Control Module Failure

The B12E8 fault code appears when there’s an issue with the daytime running light (DRL) LED module (Part Number: 4M8941475A). This module controls the power and function of the DRLs. A failure can cause:

• Flickering or dim DRLs
• One side of DRLs not working
• Warning message on dashboard
• Intermittent light function

Fault Code B121C – Headlight Control Module Cooling Fan Failure

The B121C fault code relates to the cooling fan for the headlight control module (Part Number: 8U0907463). This small but crucial fan prevents the headlight electronics from overheating. A failure can lead to:

• Dimming or flickering headlights
• Headlights shutting off randomly
• Error message about headlight failure
• Possible overheating in the control module

Step-by-Step Diagnosis of B12E8 & B121C

Step 1: Scan for Fault Codes

• Use a VAG-COM or OBD-II scanner to confirm the presence of B12E8 or B121C.
• Check for additional codes that may indicate related issues.

Step 2: Inspect DRLs & Headlights

• For B12E8:
  • Check if the DRLs are flickering, dim, or completely off.
  • Inspect the wiring harness for damage or loose connections.
  • Test voltage at the DRL module (should receive proper power and ground).
• For B121C:
  • Listen for the headlight control module cooling fan—if it's not spinning, it may be faulty.
  • Check for burnt wiring or debris blocking the fan.
  • Measure voltage at the cooling fan connector to confirm power supply.

Step 3: Reset the Fault Code & Test Drive

• Clear the fault codes using the OBD-II scanner.
• Turn on the headlights and DRLs and observe their behaviour.
• If the code reappears, proceed with part replacement.

Recommended Fix for Audi Q8 B12E8 & B121C Codes

After proper diagnosis, the most effective fix is replacing the faulty components:

• ✅ For B12E8 (DRL Issue): Replace the Daytime Running Light LED Module (4M8941475A)
• ✅ For B121C (Cooling Fan Issue): Replace the Headlight Control Module Cooling Fan (8U0907463)

Replacement Procedure:

1. Disconnect the battery to prevent electrical damage.
2. Remove the front bumper or headlight assembly (depending on access).
3. Unplug the faulty DRL module or cooling fan.
4. Install the new component and ensure a secure connection.
5. Reassemble all removed parts and reconnect the battery.
6. Clear fault codes and test lights for proper function.

Conclusion

The B12E8 and B121C fault codes in an Audi Q8 often stem from a failing DRL module or headlight cooling fan. Proper diagnostics and timely replacement of the affected components can restore full lighting functionality.

If you’re experiencing these issues, consider replacing the necessary parts, available on AUTOTEK. Need expert advice? Contact us for more guidance.

Need More Help?

If you have any questions or need further assistance, our team at AUTOTEK is here to help! Leave a comment below or contact us directly.