Replace the monovalve...buy the repair insert from the dealer (about $25 to$40, dependingn on dealer). The monovalve is located on the firewall in the engine compartment near the battery, with coolant hozes and electric wire attached. Takes about 15 min. to fix. Disconnect battery before you do it to avoid frying/shorting anything. The monovalve controls the flow of coolant to the heater core, depending on signal from climate control unit..when it fails, it stays open allowing hot coolant to circulate to heater.