SOME 2015-2017 F-150 VEHICLES MAY EXHIBIT POOR HEATER PERFORMANCE CAUSED BY RESTRICTED HEATER CORES. EXTENSIVE USE OF THE BLOCK HEATER MAY INCREASE COOLING SYSTEM DEPOSITS WHICH CONTRIBUTE TO HEATER CORE RESTRICTIONS. IT IS RECOMMENDED TO PLUG IN THE BLOCK HEATER FOR 1 2 HOURS BEFORE STARTING ENGINE WHEN AMBIENT TEMPERATURES ARE -15 DEGREES C (0 DEGREES F) OR LOWER.