I had the same problem before. The sensor has possibly come out of the mounting and fallen behind the grill.
The sensor is located on the front of the car near the fog light. If you look near the right side fog lamp you will probably see a small hole about 1/2in and the sensor is probably behind the hole.
The actual sensor is about one inch in length and pertrudes from this hole when pushed back in from the rear of the plastic grill/hole. I dont recall if it's the right or left because i'm recallng this from memory but that's what I did to fix the problem.
Friday, July 18th, 2008 AT 1:01 PM