I have a 1996 Volvo 850 with 125K miles, and the check engine light is on. My mechanic told me that the error code was P0450 which I looked up and found are the pressure sensors (specifically "Evaporative Emission Control System Pressure Sensor Malfunction").

I tried replacing the the gas cap with a new one and disconnected the battery for a half hour, but the light came back on when I restarted the car.

I know nothing about EVAP pressure sensors. Would anyone be able to tell me:
1. Where they are located?
2. How many of them are there?
3. How often do they need to be replaced?
4. Is their malfunctioning doing damage to the car? (I know that I cannot pass emissions without this being rectified)
5. How much does it cost to replace them?

Any information on this topic would be really appreciated.

Thanks,
plibicus

There is a pressure sensor located in the gas tank.
There is a evap valve located under the hood.
There is a couple of more sensors.
The first 2 are the most common problem areas.