It'll be one of the two right next to each other on the right thermostat housing (I think they're all on the right, but if not it's 1 of the 2 next to each other!) Also if it's wiring it may not show on the gauge but if you short the sending units out with the bus aired up and running the other 1 will shut the engine off when shorted! (If the shut down unit is wired correctly, which it's not a bad idea to periodaclly check both shut down units to be sure they are working! the single one on the other thermostat housing is the other 1 !) Let us know what happens! BK!