I have a 2001 Rx300 and the heater blows cold air even when set to HOT. the coolant temp indicator in dash is normal and the thermostat is working fine. I bought the car used with 125k miles in 06/2011 and only discovered the problem by 12/2011. For some reason, the heater blew hot air since 1/2012 and then now only blows cold. Can someone help me trouble shoot it? Thanks!

 

Look for threads on the air mix servo.
Like this one https://www.clublexus.com/forums/rx-...mix-servo.html

 

More than likely it's the air mix servo. It's a common problem because it's located less than an inch from the heater core lines. So, it's subject to some heat. Did you ever hear any buzzing noises under the dash near the glove compartment? If so, then that was it going bad. There are two servos. One for adjusting air to the vents, the other air temp. Both are accessible via removing the glove compartment and the panel to the left off the passengers feet area. If you see the servos working while you're adjusting temp and vent placement, then you have a bigger problem.

Check out these posts:

https://www.clublexus.com/forums/rx-...mix-servo.html

https://www.clublexus.com/forums/3946871-post35.html

https://www.clublexus.com/forums/rx-...-cold-air.html

 

Wow the 3 links contains such enormous amount of helpful information. I really appreciate it man! I'm gonna try tomorrow and see if the air mix servo arm is able to move up and down. Wow that's just easy to fix!

I do have buzzing, or clicking/rattling noise in the cabin. I tried to locate it and it sounded like it was coming from behind the cassette slot. I never noticed any noise from the passenger under dash probably I never seat there while the car is in operation.

But as I described initially, the heater surprisingly worked for about 1-2 month and perfectly fine then it suddenly stopped lately. I don't think it is something major because it worked fine for sometime. I really had a feeling that it just something need a simple twig and that's all. I'm leaning toward the air mix servo arm. I'll definitely try! Thanks man!

 

Good luck, it's not at easy fix. It's hard to get a screwdriver on some of the screws and the angle you're at. You're head is in the floor area. Aside from that, it's not bad and will save you $500 if you DIY. I went ahead and replaced both servos while I was there. It's always the air mix which is right next to the heater core lines. You'll see what I mean, it gets hot there. Speaking of that, you'll need to let the car sit for a while to keep from burning yourself while working on that servo.