Faulty brake lights when fog lights on

[Kellydavies1976]

Afternoon 🖐🏻 my 09 plate Audi a3 tdi has just failed its MOT. Apparently when my fog lights are in use my brake lights don’t work!! But they work independently of each other. Does anyone know how my husband might fix this. Many thanks 😊

Edited December 14, 2023 by Kellydavies1976

[Magnet]

Hello Kelly,

Welcome and thanks for joining. 
Simple things first - it could be a poor earth on one or both units. 
If the earths prove to be OK:- possible fractured wire/s in the flexible harness section twixt tailgate and body, or the brake light switch maybe at fault. 
Perhaps you could let us know how hubby gets on. 
Kind regards,

Gareth. 

[cliffcoggin]

That's usually a sign of a bad earth at one of the light fittings. Remove all the bulbs and clean their contacts. Do the same for the contacts within the fittings. Ensure any earth leads from the fittings are unbroken and securely attached to the chassis.

[gerri]

Hi! You must pay attention: all four brake light bulbs work normally. If you turn on the fog lights, only two brake lights come on!
If in this case only two brake light bulbs are working, they will be turned off when the fog lights are turned on!

Edited January 25 by gerri

[gerri]

7 minutes ago, gerri said:

Hi! You must pay attention: all four brake light bulbs work normally. If you turn on the fog lights, only two brake lights come on!
If in this case only two brake light bulbs are working, they will be turned off when the fog lights are turned on!

 

[Stevey Y]

15 minutes ago, gerri said:

 

Hi sorry we only got half of the posts

[cliffcoggin]

Steve,

I could see all of Piotr's message by scrolling left. Not that it matters because Kelly has not bothered to respond to the thread.

[gerri]

Sorry lads - it was my first post and I couldn't edit it. I hope next one will be better 😉

I am not sure at the moment I explained it correctly. The best way is to go to the car and check: If everything is OK and foglights are off FOUR brake lights bulbs are activated while braking. When fog lights are on ONLY TWO brake light bulbs are activated. And imagine the situation: two brake lights are broken (bulbs blown) and two are disconnected (because fog lights are on)  You have no brake lights on the back.

Regards - Peter

[Stevey Y]

1 hour ago, gerri said:

Sorry lads - it was my first post and I couldn't edit it. I hope next one will be better 😉

I am not sure at the moment I explained it correctly. The best way is to go to the car and check: If everything is OK and foglights are off FOUR brake lights bulbs are activated while braking. When fog lights are on ONLY TWO brake light bulbs are activated. And imagine the situation: two brake lights are broken (bulbs blown) and two are disconnected (because fog lights are on)  You have no brake lights on the back.

Regards - Peter

Hi thank you for that information, I did not know of that situation, just shows you you can learn something new every day,WELL DONE that man.