The van had GOOD heat all through December... I had the oil changed (genuine Toyota filter and synthetic oil) just before Christmas...I haven't used it since then, but, on the ride home from the oil change- and now with further testing - I have warm air at the rear heater vents and ""COOL" air in the front ...WHAT CHANGED ??? is the synthetic oil the problem? (i know, that's probably a dumb question-but, I always used natural oil till now). Under the van I have a warm hose and a cool hose leading to the front heater core. Could my front heater core suddenly be blocked??? It must be nearly impossible to replace that heater core, right??? Would you suggest trying a coolant flush ??? By the way, a year ago I bypassed the little valve that controls the hot water flow to the heater core (the valve under the front of the van that has the blue control cable from the dashboard connected to it) ... however, I have had good heat since then for a year, so, that probably can't be the issue. Shoot me your best suggestions, PLEASE.