What does the expansion valve on the a/c on the a/c system do exactly and when does it have to be replaced? Does it give any warning signs when it does? Does it affect the evaporator performance? Thanks, Joe

Expansion valve is meters the amount of refrigerant into the low pressure evaporator
where the rifrigerant vaporizes and absorbs heat from the passenger compartment.
If low side pressure is below normal (pressure should red 20~30 psi)then replace it.

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MB STARS Master Guild Technician,12 years MB tech
5 years independent shop owner