Parking Sensors and Trailer Recognition (w/Carista codes) - where should I be looking?

[Piplodocus]

Hi all. Got my Carista plugin today to fault-find my parking sensors on my new (to me) T6. Dunno what it means though: it's not a code in the Carista forum threads. Not really sure whether this post is best in the Carista/ODB forum or this electrics one, but either way I get:

Parking/steering assist: U111100: Function Restriction due to missing message.

Any clues as that's clearly not just one sensor out? Sometimes it pops up with the display for ~5 secs and I can see objects if close before beeping and erroring, sometimes it just beeps/errors straight away.

Also, and I don't know if it's related, but I also get:

CAN network gateway: 01517 - Trailer recognition control module: No signal/communication

I think the towbar is a Westfalia one, I think fitted by Van Haven down in Dorset years ago. So presume that used to work fine for some years at least.

But any ideas where I can look, locations, connectors to wiggle or squirt de-oxit in or the known path both or either of those run and if they likely to be related or not (they're both around the rear bumper, but that might mean nothing)? Under the front double passenger seat there's more connectors (I dunno if they're standard or retro-fitted), but I gather there may be more connectors hidden in the back under the carpeted panels Van Haven fitted ~2019ish?

It might be worth me asking Van Haven if they did fit the tow bar, but figured I'd ask here first as it may all be somewhat generic and I presume the rear parking sensors are standard at least.

I did have a few other errors, but I reset them and they didn't come back after driving round the block, so I presume they are no longer relevant (stuff to do with ABS sensors, number plate light and brake light bulbs)

 

[Pauly]

The trailer module has gone awol, I would start by finding it and checking the connections etc, probably located under one of the front seats

 

[Piplodocus]

Do you think that might be buggering the sensors due to how stuff is wired, or likely a totally separate issue?

 

[Pauly]

Function restriction/missing message is just a consequence of a fault in another module, that will disappear when the fault is cleared, start with the obvious fault and go from there

Can’t see why the sensors aren’t working, there’s nothing in the above relevant to that, a full fault/auto scan would be handy but that’s a VCDS job unfortunately

 

[roadtripper]

In my 6.1 the factory fit trailer module is under the driver's seat.

A nasty fringe suspicion occurs though - I think some "generic" tow bar kits have the option of a relay to cut off rear sensors. Possibly if the OEM module was replaced with one of those and it's failed it might also impact the sensors.

Try and trace down how the towbar is wired to get some clues.

 

[Piplodocus]

Finally fixed it!

For future reference for anyone who wants to know:
Turned out the trailer fuses under the bonnet were under quite a bit of tension after the previous owner replaced the battery. It had partly pulled one of the contacts out the back of the little extra fuse box for that trailer wiring. So I relieved some tension in that, re-cable tied it and got the contacts back in properly. OPS error stopped. Parking sensors worked.

Carista still said I had a CANBUS error to do with the trailer module. So I reset that and all is good. Error didn't return. No more OPS error beeps and I can reverse my nice shiny new T6 close to things without being paranoid I'm going to scratch it, or being miles away!

FYI: My trailer module is actually under my bonnet attached to the side of the battery bay.