FAQ
TL;DR: Most Audi A3 LED retrofits need a load resistor; 12/0.94 ≈ 12.76 Ω is typical, and "the closest commercially available value is 10 Ω or 15 Ω." Use a 50 W unit on a heat sink; coding alone rarely clears the warning. [Elektroda, SQ9MEM, post #17123418]
Why it matters: This FAQ helps Audi A3 owners who fit P21W/BA15s LEDs and still see bulb-out errors choose a safe, effective resistor setup.
Quick Facts
- Target load: Approx. 12–13 Ω at 12 V; nearest standard values are 10 Ω or 15 Ω, often in 50 W housings. [Elektroda, SQ9MEM, post #17123418]
- Thermal reality: Aluminum chassis resistors run very hot; bolt to a decent heat sink. 300 W parts run cooler but are oversized. [Elektroda, SQ9MEM, post #17123418]
- Coding caveat: Turning off cold-diagnostics in VCDS may not remove the yellow bulb icon without added load. [Elektroda, stratus84, post #17123312]
- Budget try: 25 Ω/25 W units are cheap, but the controller needs the correct load or the error persists. [Elektroda, przemek25l, post #17123518]
- Fallback: If heat or packaging is unacceptable, keep the stock P21W halogen to avoid warnings. [Elektroda, stratus84, post #17123477]
What resistor value stops the Audi A3 bulb-out warning with P21W/BA15s LEDs?
Aim for roughly 12.76 Ω at 12 V. Use the nearest standard value: 10 Ω or 15 Ω. Start with a 50 W aluminum resistor and mount it to metal for heat dissipation. "The closest commercially available value is 10 Ohm or 15 Ohm." This load mimics the incandescent draw and satisfies the bulb-out monitor. [Elektroda, SQ9MEM, post #17123418]
How do I calculate the resistor value for my LED swap?
Use Ohm’s law. Estimate the target current and divide 12 V by that figure. Example: 12 V / 0.94 A ≈ 12.76 Ω. Choose a nearby standard value, typically 10–15 Ω, and verify that the warning clears without excessive heat. Adjust within that range if needed for your circuit. [Elektroda, SQ9MEM, post #17123418]
What power rating should the resistor have?
Use a 50 W aluminum resistor for P21W-equivalent loads. It will still run hot, so mount it on a heat sink. A higher-rated unit runs cooler but is bulkier; “it’s a healthy exaggeration when it comes to dimensions” at 300 W. Prioritize safe placement over compactness. [Elektroda, SQ9MEM, post #17123418]
Why is my resistor so hot, and is that normal?
Yes. The resistor intentionally burns off power to mimic the filament bulb. It will get very hot under load. Even mounting to a larger plate may not feel cool to the touch. Ensure solid metal contact and keep it away from plastics and harnesses. Heat is expected when replacing a 21 W bulb’s load. [Elektroda, stratus84, post #17123312]
Can I use a 25 Ω / 25 W resistor instead?
You can try it, but it may not provide enough load to satisfy the controller. The system needs the correct current balance to turn off the warning. Users report cheap 25 Ω options exist, yet the error can remain if load is too low. Size the resistor to match the required load. [Elektroda, przemek25l, post #17123518]
Will VCDS coding remove the warning without resistors?
Coding can disable cold checks and reduce flicker, but the bulb icon may remain without sufficient load. One user disabled tests via VCDS and still saw the warning. Add an external resistor or use LEDs with integrated load to eliminate the error. [Elektroda, stratus84, post #17123312]
How do I wire and mount a CANbus load resistor safely?
- Wire the resistor in parallel with the LED lamp (across the lamp’s + and −).
- Bolt the aluminum body to bare metal, away from plastics and wiring.
- Test for warnings; if needed, swap between 10 Ω and 15 Ω. "Unfortunately something for something." Heat is the trade-off for clearing the error. [Elektroda, SQ9MEM, post #17123418]
Is using a resistor a bad idea—doesn’t it defeat the point of LEDs?
It adds heat and draws power like a filament, so efficiency gains drop. Some call it a “prosthesis.” Others still want LED color and appearance. Decide whether the look and instant-on behavior outweigh the added complexity and heat. If not, keep the stock bulb. [Elektroda, SQ9MEM, post #17123351]
What about legality or approval when swapping to LEDs?
Members caution about compliance and approval. Swapping light sources can affect homologation if the lamp wasn’t certified with LEDs. “There is also such a thing as approval, and the car has a Christmas tree.” Check local rules before modifying exterior lighting. [Elektroda, olekt2002, post #17123408]
Are there LEDs with built‑in CANbus to avoid external resistors?
Yes. Some automotive-shop LEDs include a built-in load or come with a resistor in the kit. That simplifies installation and can clear errors without extra parts hunting. It often costs more but reduces wiring effort. "The resistor is included." [Elektroda, mateklos, post #17123980]
If the resistor still overheats, what’s the simplest alternative?
Return to the stock P21W halogen bulb. It draws the expected current and keeps the instrument cluster happy. One user reverted after heat concerns, accepting the factory setup’s reliability. This avoids additional thermal management and packaging challenges. [Elektroda, stratus84, post #17123477]
Should I choose 10 Ω or 15 Ω for a P21W circuit?
Use the smallest value that clears the error while you can still manage heat. Many choose 10 Ω to ensure enough load. If heat is excessive or space is tight, try 15 Ω and test. Both are common and easy to source. [Elektroda, SQ9MEM, post #17123418]
