A/C evaporator
Definition
The evaporator is the small aluminium heat exchanger inside the HVAC box behind the dashboard where liquid refrigerant boils at low pressure and absorbs heat and moisture from the cabin air blown through it; the condensed water drains through a tube to the ground, which is the puddle under a parked car. An evaporator temperature sensor keeps its surface just above 0 °C so it does not freeze into a block of ice, by cycling the compressor or reducing its displacement. Leaks develop from corrosion by condensation acid and road salt drawn in through the fresh-air intake, and are hard to find because the core is buried: sniff for refrigerant at the drain tube or centre vent with an electronic detector, or look for UV dye in the drain water. A blocked drain tube backs water up into the footwell carpet, causes fogged windows and a musty smell, while a dirty, mould-covered core causes smell on start-up and poor airflow. Replacement means removing most of the dashboard, so confirm the leak carefully first; always renew the expansion valve, drier and O-rings at the same time. Codes: B1xxx HVAC codes for the evaporator temperature sensor (open, short, implausible), manufacturer codes for evaporator freezing protection; leaks set no codes.