False light warning for trailer

[alogbe]

I have a 2007 A6 2.7TDI with a factory-fitted towbar. For several months now, when a trailer is connected, I've been getting warning signals on the dashboard display telling me that a trailer light isn't working; sometimes it just says a rear light, more often it specifies the right-hand indicator. It typically happens only after a few miles on the road, not when I start up.

But it's lying. I've stopped to check it many times (leaving the engine running and actually looking at the back of the trailer) and the lights are always working. I've cleaned the connections and the bulb-holder contacts, but it makes no difference.

Can anyone help?

[kite]

put a new bulb in.
the system is quite sensitive and can give you a warning before a bulb blows.

[alogbe]

Thanks for the suggestion. Unfortunately I now think the fault must be in the car, because a couple of days ago I fitted a bike carrier (which I haven't used for a while) and found that I'm getting similar messages about a "fault" in the carrier's lights.

If I'm right and it really is the car, does that mean replacing something expensive? Obviously I'll talk to a dealer if necessary, but I'd like to be prepared.