I bought a used RX 350 with 130,000 miles. It was in pristine condition. About a month after the purchase the AC stopped working. Had it charged with dye and in 2 weeks it was blowing warm. No sign of leaks in the engine compartment using a UV light. Sniffer did not detect a leak in the cabin. Mechanic says it is probably the evaporator core. $1300-1600 to replace because the entire dash has to come out. I am going to get a second opinion, but in reading about it most people say this is highly unlikely. Where else can you look for leaks that might not show under a UV light or sniffer? What if I charge it with a refrigerant (R-134a) with a leak sealant? Any other ideas? Thanks. 
Get a second and third opinion on this. A/C problems can become expensive if not diagnosed properly.
ac worked for 1 month after purchase and than it dropped to 2 weeks? seems like leak is getting bigger. i would inspect bottom of condenser for obvious damage. and or dye seepage. its hard to get close to bottom of condenser with all the shielding and plastic coverings.
Time to bring in a pro, get a second estimate. They shouldn’t be guessing that it’s the evaporator, they should be able to find the leak.
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The problem might be with the expansion valve.
The expansion valve is what controls the pressure drop into the evaporator. This is what makes it cold.
If the expansion fails, nothing gets cold.
Bring the vehicle to an AC shop so the low/high side pressures can be checked.
Tester
Second the idea of getting this problem diagnosed by an AC specialist shop. You may not even have a leak. Other problems besides leaks can cause the AC to blow hot air. You definitely should not add refridgerant unless you are certain the AC charge is low. Overfilling may damage the entire system and require replacing nearly every part. Suggest you read through this 4 part article on automobile AC for more info.
RX350’s and ES350’s have been known to have evaporator coil leaks. I remove the blower motor and inspect the evaporator with a borescope, there is usually an oily mess in there.
Thanks for your advice. I have called a few other shops, but all are general auto repair. Right now, the air is blowing cold. 2 weeks on after refilling for the 3rd time. When it stops blowing cold I will try to find an AC specialty shop. I don’t want just anyone taking my dash out. I mentioned using a borescope and the first shop said they can’t do it on this car.
It is definitely leaking coolant though. Both times it was refilled it was low.
I absolutely agree
I often use my video scope to look at components which are not readily accessible
I’ve spotted a few leaking evaporators this way. Thus, I performed a proper diagnosis
dye, sniffer, video scopes, whatever it takes
As for other possibilities . . . I believe nobody has yet mentioned the evaporator temperature sensor
I don’t work at a Lexus dealership, but I’m a little skeptical. I’ve had pretty good luck, on various brand vehicles. It’s not always easy, and sometimes you have to remove more than just the blower motor
Thanks. That was helpful to understand the modern AC system. After reading that and watching a few videos I will not be adding any sealant. It worries me that a prior owner may have done so.
As of today, the AC is still working. It’s been a little over 2 weeks. So it is not an emergency.
One thing I am going to do is replace the cabin air filter. There is a message in the display that it is time to change it. Seems pretty easy to do.
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