BMW X3 Trailer Wiring Causes Parking Sensors And Auto Braking Issues

Updated 12/22/2025 | Published 12/19/2025

Question:

I have a 2018 X3 M40i that I installed a hitch on and the TK77 harness. When I plug in to the trailer it’ll temporarily enable the trailer camera and the car sensors go nuts, and if I were to try to go into reverse, the car will try and auto stop itself - I assume becaues it thinks the trailer is an object. I tried turning the sensors off and the car still acts oddly with the trailer plugged in. Once the trailer is unplugged, I no longer have the trailer view I assume it should stay for easy backing up and attachment Does something need to be coded? Is something faulty with the harness?

asked by: Chris V

Expert Reply:

Hey Chris, thanks for reaching out. What you’re experiencing is normal behavior on BMWs when using a non-OEM trailer wiring setup. The # TK77FR harness provides trailer lighting only and does not communicate with the vehicle’s CAN bus or place the X3 into a true trailer mode. Because of that, the vehicle briefly detects a load and may momentarily show the trailer camera, but it never fully disables the rear parking sensors or automatic braking. As a result, the car still interprets the trailer as an obstacle and reacts accordingly, especially in reverse.

There’s nothing faulty with the harness. To get proper trailer behavior, including consistent trailer camera operation and suppression of rear sensors and auto-braking, the vehicle needs OEM-style trailer coding. Without that coding, BMW’s safety systems will continue to act as if the trailer shouldn’t be there.

expert reply by: Jesse M

Chris V.

12/19/2025

Ok, so I should speak with my local dealer about coding? It’s a bit difficult to backup with the trailer attached. Thank you!

Jesse M.

12/19/2025

@ChrisV Yes that is definitely something a BMW dealer can help with.

Paul H.

12/22/2025

@ChrisV Hi Chris, have you tried turning PDC off while backing up? That resolved the issue on my X3. The PDC sensors simply “see” the trailer behind the X3 and think you’re about to back into something and immediately engage the auto brakes. That’s all that’s happening, no coding required, just turn off PDC while backing up. Turn it back on after you’ve finished backing up.

Chris

12/22/2025

@PaulH honestly, I don't remember. I haven't hooked it up recently. I thought I turned all of the sensors off, but I'll have to double-check, it's always possible I missed something.

Paul H.

12/20/2025

So, I have this trailer lighting harness on my 2018 BMW X3 xdrive3.0 with a hitch that I installed myself. So, NOT the BMW trailer option. First time I hooked up a trailer and connected the wiring, I couldn’t back up because the X3 would automatically apply the brakes, probably because the PDC sensors thought I was about to back into something (the trailer). Solution- I simply press the PDC button on my center console to disable the PDC system and cameras, backing up is no problem. With a trailer behind the car there’s no need for the cameras anyway because there’s nothing to see. The side door mirrors are sufficient for making sure you’re not going to hit anything.

Jesse M.

12/22/2025

@PaulH Hey Paul, thanks for sharing! Great to know the backup issue can be solved by just changing the settings. Did the car do anything else weird with the trailer hooked up after disabled with PDC system? We have had a few people report back the vehicle still acts funny while towing.

Paul H.

12/22/2025

@JesseM Hi Jesse, no, the car works because exactly as it’s supposed to because you can either just turn PDC on as soon as you’re done backing up, in fact, on my car, it automatically turns back on as soon as I shift the transmission from Reverse to Drive. I’ve towed trailers many times with my X3 and haven’t noticed anything else weird other than while backing up, which is easily resolved by simply temporarily turning off PDC.