I have a 2012 HD Tri-glide. I have recently taken it to the dealer with a problem they cannot seem to diagnose. The problem is the horn cover on the left side of the trike seems to have developed an exhaust stain on the chrome cover. The dealer has no idea why this keeps showing up. I took it to them thinking the trike has a blown head gasket on the rear jug. They did a cylinder leak down test but found that the head gasket was OK but the exhaust stain keeps showing up on the back side of the horn cover. I noticed on the front head behind the horn bracket there is a brass fitting that goes into the head. The dealer is telling me that this is a temperature sensor. I do not know what it is since I do not have a manual to tell me what it is. Is it a temperature sensor or a compression release valve? Does anybody have an idea of what it may be and why there is an exhaust stain? I do not think there are very many places that the exhaust can come from. It is either on the back or front of the heads.