Home >> Porsche >> 2003 >> 911 Carrera 4S, Standard >> Repair and Diagnosis >> Heating, Ventilation & A/C (HVAC) >> HVAC Control Systems >> Heater, Diagnosis - 911 CARRERA (996) >> Fault code 21 - as of MY 2002 >> Outside temperature sensor - short circuit to B+/short circuit to ground >> Possible cause of fault