Sensors on '99 5.7L AC system

The truck is a '99 2500 Chevy pickup w/ 5.7L and AC. The AC compressor stops working at will, but when working is producing cold air. The problem is intermittent as one time it will work fine for miles and others it stops
cooling within a short distance, not necessarily completely up to temp, and running the compressor. The person has not tried stopping the engine for a few seconds and restarting to see if this resets the sensors, etc.
My question is what are the two sensors sensing on the AC system, one going into the compressor on the top (HOT Side) and one going into the silver canister on the firewall (COLD Side). If one of these is on the fritz will it stop the compressor? What do they sense? Can they be replaced without discharging the system?
Thanks, Kevin
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The High side sensor (in the compressor) cannot be removed without evacuating the system first. Problem is most likely the low side switch this switch is mounted on a valve so it CAN be removed without removing the system charge very little refrigerant will be lost in the replacement.
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Thanks guys for the info. I will pass it on to the party that owns the truck and hopefully he can get it repaired cost effectively!

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