[brcidd]

Was the compressor running when you saw the 40 psi? If not, then 40 psi is your static pressure (compressor not engaged) and your system needs 47 psi to turn on the compressor. So if 40psi is without compressor running, then you are extremely low on refrigerant and compressor will come on once pressure rises above 47 psi as you add refrigerant.

If above is the case, then your pressure transducer is protecting the compressor by disabling it, the last thing you want is the compressor to run with little or no refrigerant which carries the oil needed to lubricate the compressor.