Just replaced one today. Simple test is to remove the power connector from the valve - this should open the valve & give full heat. Beware the plastic parts of the valve become brittle with age and are easy to break. You need to bleed out the air afterwards - with a COLD engine, remove the pressure cap from the expansion tank & run for 2min MAXIMUM, topping up expansion tank as required, then replace the cap.
Handy Spanner, January 2010