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Audi A3 selection of the CANbus resistor - How to choose a resistor for LED bulb

stratus84 13788 11
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  • #1 17123312
    stratus84
    Level 10  
    Hello, I want to change classic bulbs to LED bulbs, unfortunately there is a problem with burnt bulb error. What was possible, I turned off the cold type test and other functions via VCDS. I link what I bought:
    LED bulbs: https://en.aliexpress.com/item/Ground-CREE-80...-24V-car-Reversing-light-bulb/1070476831.html ? spm = a2g0s.9042311.0.0.w9NHbo
    PHILIPS 21W resistor: http://sklep.led-car.pl/rezystory-canbus-do-z...-lumina-21w-do-zarowek-led-p21w-p21-5w-py21w- error-reseter-error-computer-resistor-error-error-canbus-.html

    Everything works fine, there is no information about a burnt bulb but there is a high temperature, I accept a warm resistor but this one is hot ... I screwed it to a larger cold plate but it did not help. I am asking you for help in choosing the right resistor, what ohms and other values would you suggest? Links to the proposed resistors are welcome :)
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  • #2 17123351
    SQ9MEM
    Level 29  
    And what is the point of buying LED bulbs and then inserting a prosthesis in the form of a resistor so that the error does not appear?
  • #3 17123387
    stratus84
    Level 10  
    Unfortunately, I have no choice, the LED is lit without a prosthesis, but instead of an electronic speedometer, I have a yellow bulb icon on the desktop. I personally do not like the classic bulb and it is not as visible as the LED, I mean visibility on the road, the visual effect is also a plus for LED in my opinion.
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  • #4 17123400
    osiniak75
    Level 35  
    I look and my eyes do not believe so much money for two resistors, well estimate, what country tuning must cost. Can these bulbs be used in Poland?
  • #5 17123408
    olekt2002
    Level 31  
    stratus84 wrote:
    I personally do not like the classic bulb and it is not as visible as the LED, I mean visibility on the road, the visual effect is also a plus for LED in my opinion.
    The bulb is not meant to be liked, it has to do the job. You improve engineers?
    There is also such a thing as approval, and the car has a Christmas tree.
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  • #6 17123409
    stratus84
    Level 10  
    Rural tuning? Without your balls, you must have made a mistake, rural it would have been if the car was decorated with some crap, flashing nonsense. I do not change anything in the car except to change the color of the light to match the bixenon ... Yes, I regret the PLN 100 for these resistors but they were supposed to be ok. If the help on this forum is to rely on this way, thanks a lot!
  • Helpful post
    #7 17123418
    SQ9MEM
    Level 29  
    Ok. The parameters on the website show that the resistance of the resistor sold is 12 / 0.94 = 12.76Ohm. The closest commercially available value is 10 Ohm or 15 Ohm. The one you bought has a power of probably 50 W (e.g. something like this http://www.sklep.monster.pl/10r-50w-rez-z-radiatorem-alu-canbus,4,44483,65653) to get less the heater needs to be a piece of a decent heat sink or a more powerful resistor, for example, http://www.sklep.monster.pl/10r-300w-resistor-z-radiatorem,4,44480,44181 but it's a healthy exaggeration when it comes to dimensions . Unfortunately something for something.
  • #8 17123442
    osiniak75
    Level 35  
    Don't be angry anymore, you'll understand it someday.
  • #9 17123477
    stratus84
    Level 10  
    osiniak75 wrote:
    Don't be angry anymore, you'll understand it someday.


    I already wiped my tears calmly. And will I understand, I don't know what to understand? I figured I would get some information here, but most of it is off topic, plain spam.

    SQ9MEM wrote:
    Ok. The parameters on the website show that the resistance of the resistor sold is 12 / 0.94 = 12.76Ohm. The closest commercially available value is 10 Ohm or 15 Ohm. The one you bought has a power of probably 50 W (e.g. something like this http://www.sklep.monster.pl/10r-50w-rez-z-radiatorem-alu-canbus,4,44483,65653) to get less the heater needs to be a piece of a decent heat sink or a more powerful resistor, for example, http://www.sklep.monster.pl/10r-300w-resistor-z-radiatorem,4,44480,44181 but it's a healthy exaggeration when it comes to dimensions . Unfortunately something for something.


    Thanks for the help and information, for the moment I'm staying with the classic P21W.
  • #10 17123518
    przemek25l
    Level 29  
    On the Allegro I saw that the 25 Ohm / 25W mount is not expensive so you can try it.

    Added after 3 [minutes]:

    Seemingly better because less electricity. On the other hand, they install resistors on which you need to load, i.e. lose the amount of current, because the balance for the controller must be correct so that the error does not shine.
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  • #12 17123980
    mateklos
    Level 23  
    Hmm, you can buy automotive shop LEDs with approval for our Polish roads . The cost is more, but the fun is when you only have the installation and connection, because the resistor is included.
    And so you can download applications on the smartphone, types of electrodes, something and calculate what the resistor should be

    Moderated By CameR:

    The approval covers the lamp together with the light source.
    Reg. 3.1.11. Don't post messages that add nothing to the discussion. They are misleading, dangerous or do not solve the user's problem.
    Reg. 3.1.13. Take care of linguistic correctness and follow the rules of netiquette. Do not post messages that make it difficult to deduce what their author wanted to convey.

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Topic summary

✨ The discussion revolves around the challenges faced when replacing traditional bulbs with LED bulbs in an Audi A3, specifically regarding the need for a CANbus resistor to avoid burnt bulb errors. The user has attempted to disable certain functions using VCDS but still encounters issues with high temperatures from the resistor. Various participants express skepticism about the necessity of using resistors, while others provide technical advice on selecting appropriate resistors based on resistance and power ratings. Recommendations include using resistors with values around 10 Ohm or 25 Ohm, and suggestions for purchasing resistors with adequate heat dissipation capabilities. The conversation highlights the balance between aesthetics and functionality in automotive lighting modifications.

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

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?

  1. Wire the resistor in parallel with the LED lamp (across the lamp’s + and −).
  2. Bolt the aluminum body to bare metal, away from plastics and wiring.
  3. 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]
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