You've got several proximity switches on the boom . You have two boom counting proximity switches that travel with the the boom and the boom home proximity switch located up high and a bottom boom proximity switches. It's probably one of the count proximity switches. Look on the
door and locate these functions on the input card. One count. bottom limit, and the boom home is on input card 2008 and the other count switch is on 2010 I believe. The corresponding light on the input card should light up when the target is in front of the proximity switch and off when there is no metal in front of it. Again I'd look at the count switches first but check the boom home. If you have to you can run the unit without the bottom limit so you can rob one from there if in a pinch. It will not successfully pass a boom test though without the bottom limit.
ETA: You can do a boom count test in the tech menu. If it tests there OK but not while the wash is spraying its possible you've probably got a spacing issue on the proximity switches as the boom is being influenced by high pressure.