My AC is not blowing out cold air. Just had it serviced last summer. Used it for 3 months. Was told there was no leak. Car has 100,000 miles on it. How do I fix this myself?

no car told at all year yes. sad face and sorry no fuse to tell for just that 1 reason, and no engine told as fuse boxes vary buy ENGINES.

sorry 100,000 models of cars and all that,
now the generic answer and posted here vast times , 1 more.
first off what does UNIT mean to you, me zero.
AC dead , NO COLD air summer, are you in the FLORIDA keys?
there are to macro causes,
1: the R134 loop is dead. or weak.
2; or air distribution system (cab blower etc) is failing. or weak.
and we do this in that order, LOOP first. and using gauges for
low side and high side to be in spec, per the manual.

the dead compressor has these causes.
1: R134a leaked out as ALL DO and the high low Freon switch is tripped open, saving destruction of the compressor total, and worse.
2; the blower is dead in cab, if dead (no 12vdc to ) the AC is disabled so the core can not ICE up and brick out (more bad that)
3: the BCM runs this system, with aid of the controller AC.
4: fuses blown to AC clutch
5: relays blown to AC clutch and its fuse.
6: 12vdc makes it to clutch is a bad clutch or omg seized compressor.
all it takes is a volt meter and service manual on the car.
and even scan the PCm/BCM and it tells you OMG blown fuse X

the car has glove box guide there, turn to chapter fuses. read that.
or if lost that, buy one in usa the guides are free, PDF. and easy to find googled in seconds,
the service manual is here, (I too must pay for top manuals)

https://eautorepair.net/Marketing/Default.asp

the clutch also goes OFFLINE (on purpose)
if the BCM controller sees Evap core IS up (a sensor)
or if outside air is too cold so try defrost mode.
the BCM also cuts AC off line if engine or transmission shows overheating NOW. saving engine for destruction (warped heads no fun)
some cars see ENGINE in PCM limphome mode ,cut AC dead.
those are all called now, today, permissive's.
seen same thing when throttle by wire fails.(LIMP) car is a slug.
so if CEL lamp is on the PCM must be scanned first.

good luck , tell more get more
no car told at all so........a hard nut case here.(nut to crack)

hey they all work great new, but not now, the endless story.

no car told no year told no model card told (fun?)
hot air is actually outdoor air? that is 100F here. USA?>
as you do know the HVAC on cars can in fact fail and stick in heater mode.(damper failures)
rules of the game AC. (short list I have huge list, ask as we do tests)
1: the dampers must work correctly in the heater box.
2: the compressor must spin (and does)
3: the refrigerant must not be over charged, ever or it will fail.
ample is not the correct word, the correct words are
the Refrigerant it put in by weight. (an exact measure ask any HVAC shop and hear those words)

5: engine over heat sensor shuts down AC. (not this case)
compressor spins, so. no this.
6:blower is dead, in AC mode( not this case it blows)

7: the condenser fan up front is dead, fails parked but AC works great at highway speeds. 50mph (+)

on my car the ac/heater has self test mode, to be sure all damper motors work correctly so we do that first. (mine is JEEP)
most modern cars have 3 dampers, hot cold and recirculate.
use recirc mode for AC for best cooling.
some AC have hidden in dash filter that packs up in lint and can not pass air now.

on one car a rodent got into the fresh air vent and went deep into the heater box nest built, but was in front of AC evap core but blocked cold air from evap core. ( what a shock)

why not take it to any ASE shop and as for diagnosis.
pay the man to do al tests on AC
and if you over filled it , bingo you caused it.
the modern R134a , hates overfilled. !! repeat for effect.

Are you measuring from the Low side with the AC ON? Because you need to do that. (and out door temp tables read that)
the factory service manual tells what it must be.
and no 2 cars are the same due to volume the same.
classic case of fail. (sad story time)
R134a leaks out, (so many ways on cars)
and owner buys the $12 Walmart kit and overfills. it. and fails.
or worst case 2:
if pressure was zero , key off and on. the loop now has water inside the loop (called humid air) and that makes it bad after a refill.
the water is the killerr here.! must be vacuum pumped down first this case.
why not get it serviced by pros.
they do a full leak test with dye. do it now.

sounds like a faulty sensor

Your leak may not be coming from the outside. The AC has an evaporator tray that catches the humidity pulled from the air in the car drains it to the ground.. Sometimes the drain gets plugged up. Clear the drain and clean the tray. Sometimes that situation is accompanied by a nasty smell.

If the ac compressor is running and the line is cold, it could be a blend door under the dash that is not cutting off air flow thru the heater.

To clarify - - if the truck has rear and heat, you have two heater cores and two evaporator cores. The evap cores carry the freon.
If the leak is in the evap core in the front, the price is probably right.

If this was happening to me.... I'd goto an airconditioning re-gas centre and get them to check for gas leaks. In my old commodore the Evaporator Core was leaking and causing some of the issues you have mentioned.

you need to turn on the a/c and watch the electric clutch engage on the front of the compressor. Sometimes, when the clutch gets hot, it won't lock up and lock the compressor to the pulley. You might be able to adjust the gap closer, or replace the clutch. Might have to pull the comressor to do this. If you had a leak, it never would have strated working.

there are several possibilities. when was the last time this van was tuned up? new plugs and an air induction cleaning can work wonders. depending on where you live, if its cold, fuel economy can drop significantly in the winter months. my own car gets aprox. 100 - 120 KM less per tank then in the summer time.

The most probable cause is that you have a leak in your A/C system..and it needs to be serviced with R134A and a can of red dye added so the leak can be found..i hope this helps ya...Keith