99 E320 heat has to be on HI to heat
 

I haven't found this particular problem while doing searches. My AC is fine, but my heat is binary: it does not heat at all unless the thermostat is set to "HI". Then it gets hot. Otherwise it feels like fresh air. Also, my center vents put out cold air, but only sporadically. Other threads have described symptoms that are somewhat similar, and discuss the monovalve or vacuum actuator. Is that where I need to go? For what it's worth, I had my local MB dealer diagnose this problem during the last service call before the car went out of warranty. They said there was no problem and that the center vents only blow cold air so there was nothing they could find wrong with it(?). Any help would be appreciated.

I can send you some documents just email your vin via the website

Check the interior temperature sensor aspirator fan. It's located by the dome lamp, and should hold a 1cmx1cm piece of paper to the opening by the suction of the fan.

documents sent