Rusty gunk is enough to make it "stick". Mine would randomly operate, then get stuck on full heat (not nice). New valve cartridge has worked fine since replacing it.

Make sure the coolant is nice and clean before you install the new cartridge. If it's rusty or in poor shape, use the opportunity to flush and refill the cooling system. It'll be appreciated by your engine and your climate control system.

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)