We returned to our clients home for the unit not cooling properly. We immediately proceeded outside and found the unit to be in a low pressure lockout. Our client had not yet placed the air filter and we discussed how the unit needs an air filter to add some restriction and help the unit preform its proper function. That without the filter the air moves across the coil too quickly and trips the outdoor safety. We reset the outdoor unit and it ran properly for 20+ mins. It then shutoff. I assumed it was the float switch which I had checked but the other technician was outside and corrected me saying it had shutoff prior to me checking the water switch. We then further proceeded to test the outdoor unit. We found that the outdoor defrost control board was getting all the proper voltage to the board and into the safeties but it was not sending signal to the contactor. I immediately showed this to our client and we discussed how this was an intermediate issue and we were lucky to catch i