Low charge unit would not run, had it checked.it had a low charge but no leak was found. it was recharged and ran good for 3 to 4 months. now it has a low charge again.

The compressor needs to run, does it run? If it runs and the outdoor radiator is not producing heat, then it may be low on refrigerant. The compressor refrigerant lines, the skinny gets hot and the thick line gets cold under normal operation.
To recharge the unit, it needs to be leak checked and repair any leaks found, then recharged. The refrigerant type used needs to be known, a manifold gage set is used to diagnose and recharge.
If you have a window unit, you should have it serviced or replaced.

You may be able to hear the leak but the chances you'll see it are slim to none. Also if it's a bad leak the freon could all be gone now. Usually when a unit is low on gas the compressor will short cycle. Otherwise your unit might just need a good thorough defrost. If the compressor is running and not short cycling my money's on the defrost.

Hello, anytime the unit does not blow out cool air I like to start with checking the airflow, check the filter and the evaporator coil for a buildup of dirt. If that check out ok then either the unit has a refrigerant leak or the compressor is not operating if the compressor is running than the unit is low on refrigersnt and can be recharged. In order to recharge the unit a piercing valve must be installed on the low side of the system, most window units do not have valves so one must be added to be able to charge the unit.

Sealed systems can leak at the evap and/or the condenser. Charge ports on the compressor can develop a leak too. If a drier was sweated in and the Schrader valve was left in when it was being installed, there will be a leak there. Leak detectors now a days are electronic sensors and are more sensitive and safer to use.

P4 is a overheating compressor code in the outdoor unit. You probably still have a leak. Have your brother remove the remaining freon and pressure/leak test with a charge of nitrogen. I'f its leaking out this fast you should be able to find the leak with a soap solution. Once you've found the leak, pull a vacuum and flush with nitrogen before recharging the unit.

What is the voltage you are reading? Is it below 13.2 or so? Also have you checked the current with the engine off to see if something else is working that should not be on. I ran across this twice while working on a truck and car. A relay that was wore out keep trying to close and would drain the battery every couple of months. Sometimes you can hear them clicking or buzzing. Touching them sometimes they are warm or real hot. But that is not a sure sign it is bad. In my Trailblaze they get hot but are ok. Another thougth is the belt on your alt, correctly set to spin it at the right voltage. I have ran across this many times lately with serpatine belts.

Sounds like you lost the refrigerant charge. Low pressure switch will not let compressor run. Look for oil around the condenser tubes. If you find it, this is probably the leak source.

The high side/low side pressure equalization you describe is normal for a compressor sitting idle.

If you have an ohm meter you can check the compressor windings for opens and shorts. If your compressor is 3 phase as you state (this would be very unusual for a residential unit) you should have continuity across all 3 phases and no continuity to ground. Take the compressor out of the circuit before you check.

If the compressor does have crankcase heat, it should have been left on for at least 24 hours before starting the unit. Another cause for failure could be overcharging which could have resulted in refrigerant slugging back to the compressor.

Does the unit have any low or high pressure control safeties? If so, check them too - some have resets. You said the condenser fan ran so it's not likely the breaker blew, but check anyway and also check to make sure you have available power on all 3 phases.

The unit is probably low on gas again. If it lasts a month the leak may be difficult to find, as it may be a small leak. Those units don't hold a lot of gas. Have someone do a nitrogen leak check and fix the leak. If no leak is found, have them install a UV dye to help find the leak. :)