Problem with a/c blowing warm air after freon was placed in 3weeks ago and service engine light is on

Yes, It has to be done by weight. Now you need to take it to a vehicle AC place, have the refrigerant sucked out of it onto a recovery machine that also creates a vacuum and the correct amount of gas added by weight. They use subtractive scales to weigh the gas cylinder. It also has to be the right type of gas.

Your AC system is a closed system, so the fact you have to put more Freon in, means there is definitely a leak. I would check it out with a mechanic.

Sounds like your HAVC pressure switch is going out cutting off prematurely or the system is overcharged and shutting off to prevent damage.

Just a little low on freon can make it get too cold and it freezs if you in position to check the freon level you'll find with the system on high and the engine reved up the low pressure is below 25PSI. fill it to reved up is above 25 and your problem will go away. Low on freon will make it cold enough to frezze up and then warm up as the air does'nt go though the ice. it'll work good as you start up till it frezzes and after it's stopped up with ice it blows warm (it's really too cold). the more freon in the system the warmer it'll all blow because all the pressures are higher and it's the difference in pressures that make it cold.

It does not have one, and if it did it would not make it blow warm air, just less air. Have the freon charge checked in the AC system for proper pressures, would be a good place to start, & check to see if the AC compressor is kicking in. I hope this helps.

I have a feeling that no one is checking the blend door in the plenum.

This one is a difficult one to say. You could have a bad ac unit on the cars engine. Also when it goes back to 72 degrees your car is just displaying the temperature inside the car one of the features of a digital adjustments on these types of cars. Also you could have a leak and therefore not getting the cold air you wanted.

You have a leak in the AC system, have it tested and repaired.

Ok I've been around too long...R134 (freon replacement product)

Your system has R-134 Freon in it. the 134 is made up of smaller molecules than the old R-12. The R -134 permeates through the rubber parts of the system. If the system gets too low on Freon, the compressor will stop coming on.(low pressure cut out switch). Have the system serviced with an A.C. machine that can vacuum, measure the amount of Freon in the system, and add the right amount. The machine can also inject die. Later the die will show up under a black light.This will show any leaks you may have.