I believe Mr. "All Who Wander..." is correct. Every time I hook up the horse trailer, I run through the light test. Tail lights (markers, running lights) have to be tested manually with the headlight knob. Most of the time it is dark when I end up hooking up to the horse trailer...my wife...
Now I see a pattern...I too received the "left turn signal" error with no trouble with the light (LED) on trailer flashing away happily...something to keep awareness of and report if it pops up again!
When I received my truck, in the back seat was the sensor, and...
...wait for it...
...in the glove box was the pack of stickers.
I installed the sensor on the new horse trailer as the stickers never worked well on the old one.
I do all this for my wife, she just can't get the hang of...
I just picked up the wife's new horse trailer. It has a RV battery to power the lights and other nifty things.
The trailer dude stuck his tester into the 7 pin on my truck, it didn't indicate any power was flowing from the 12v line to charge the battery. (All the other connections were...