Problem with tail lights.

[Al_B17]

After a cold start the outermost bar on the passenger side is stuck on the braking brightness whenever the car is turned on. I have tried unplugging the light and disconnecting the battery for about 20 minutes but neither worked unfortunately. I have read in some places that the only way to fix it is a new tail light but I also read that some people bought a new tail light and it still had the same problem. Anybody have any tips of how to fix this? Much appreciated

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[Cobra Jet]

After a cold start the outermost bar on the passenger side is stuck on the braking brightness whenever the car is turned on. I have tried unplugging the light and disconnecting the battery for about 20 minutes but neither worked unfortunately. I have read in some places that the only way to fix it is a new tail light but I also read that some people bought a new tail light and it still had the same problem. Anybody have any tips of how to fix this? Much appreciated

If you tried a battery reset and that did not fix the tail, then it is most likely a bad BCM.

There is not only an external BCM for the rear tails, but within the taillight assembly itself there’s electronics that control the LEDs.

If the vehicle is out of warranty or the part won’t be repaired under warranty provisions - the 2015-2017 rear taillights can be had for $300-$400 for a pair on this site in the classifieds section OR expect to pay about $150-$300 per light depending on who’s selling and on what platform. Also you can look these up on eBay - there’s a TON of them for sale ranging anywhere from $150 each upwards of the Ford retail of $800-$900+.

Don’t pay retail for any S550 rear taillight....

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This thread may help you out, as I have never messed with Forscan (yet). The post is from a guy in Switzerland, but it sheds some really good info on the BCM and tail wiring.
https://www.mustang6g.com/forums/threads/disabled-sequential-turn-signals.120768/#post-2523035

This thread is from a guy in Germany, but the responses shed more light on the association of the taillight to the BCM.
https://www.mustang6g.com/forums/threads/programing-taillights-mustang-s550.118792/