That sounds like a good place to start. It could either be an electrical problem with the valve controls, or a mechanical problem within the valve.

Remember that if the monovalve is de-energized, it defaults to the open (passes fluid) mode. Also remember that the control element for the monovalve is in the climate control box and it switches the grounded side of the circuit. So just because you measure 12 V at the valve does not mean anything.

__________________
The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech