It gives the engine control module the green light to start the engine. If a vehicle’s DEF head isn’t working, it will not run correctly.
Passenger vehicles, big 18-wheelers, and motorhomes made after 2010 with diesel engines will have DEF tanks that drivers need to keep filled with diesel exhaust fluid.
Sadly, DEF head failures have become increasingly common, especially since 2017. The EPA made a major adjustment to measure the concentration of the DEF.
While the DEF sensors initially seemed to fail for no reason, it was later determined it was a heat issue.