Proving the DUO valve bad is a SIMPLE task that takes less that 5 minutes.

Simply block off the heater supply line with needle nose pliers.. I have special hose type pliers that won't damage the hose. If you have a piece of rubber you can use it between the pliers & the heater hose.

__________________
MERCEDES Benz Master Guild Technician (6 TIMES)
ASE Master Technician
Mercedes Benz Star Technician (2 times)
44 years foreign automotive repair
27 Years M.B. Shop foreman (dealer)
MB technical information Specialist (15 years)
190E 2.3 16V ITS SCCA race car (sold)
1986 190E 2.3 16V 2.5 (sold)
Retired Moderator