The backup sensors in my 2007 F150 had gone bad. Every time I shifted to reverse, the sensor would sound the alarm regardless of what was or wasn't behind me. Picked up 4 new sensors from Amazon for about $30 for all four. Just installed them and the system is working fine now. Just out of curiosity I checked with Ford to see what the Ford OEM parts would cost. Ford wanted $165 per sensor. That's $660 for 4 sensors, before tax. I understand that OEM parts for better in some cases, but a $630 difference is ridiculous.