What's the procedure to run an AC module self diagnosis? For other Lexus models i see you have to press at the same time Auto + Recirculate. However, the IS doesn't have an auto button (just a position on the dial).

Problem is i'm getting no heat (or extremely little heat) in the winter - and the AC is blowing lukewarm air in the summer.

The interior temp sensor is working fine - tested it with a heat source, the temperature servo motor starts moving (as it should - i think...).

So to the temperature motor / stepper / servo or whatever it's called is working fine (i even tried removing it and moving the lever attached to it by hand - still no heat, no matter the position).

The ac mode position servo is also working, the recirculation servo is working and so is the fan.

The heater pipes - one of them is not insulated - could not check properly if it was hot or not, but the other pipe (not insulated) was hot.

Trying to diagnose this thing before i start tearing the dashboard apart... any help would be appreciated.

Hi, I would start by feeling both top and bottom radiator hoses both should be hot, but bottom should be cooler than the top hose.

I would then check the car gets up to temperature and gauge stays in middle even when driving. This rules out thermostat. 

Then I would check the heater matrix pipes going into the cabin from engine bay. I suspect the heater core needs flushing to gain heat back in the heater. This is most likely the cause. 

Of course, the most obvious of all is make sure coolant levels in both the expansion tank on back of radiator and the radiator are full. 

46 minutes ago, Dean1309 said:

Hi, I would start by feeling both top and bottom radiator hoses both should be hot, but bottom should be cooler than the top hose.

I would then check the car gets up to temperature and gauge stays in middle even when driving. This rules out thermostat. 

Then I would check the heater matrix pipes going into the cabin from engine bay. I suspect the heater core needs flushing to gain heat back in the heater. This is most likely the cause. 

the thermostat is fine, the gauge stays in the middle all the time (once the engine heats up).

tell me more about the flushing, how is that done? is it a diy operation or will i need special tools?

To flush the heater core, you will need two clear hoses (clear just to see the colour change in the fluid) connect both hoses onto the heater core at the back of the engine bay. One hose into a bucket, the other hose connected to a garden tap/hose on low pressure. Flush the core one way, ensuring fluid goes from brown to clear, and is free from any dirt/debris. Then switch the hoses around on the heater core and flush the other way.

Once all is clear, pour anti-freeze into one hose and make sure you fill until it comes out of the other hose, So you know the whole heater core is filled. 

Put the hoses back on the heater core in the same orientation you took them off.

turns out the levers and stepper motors are working fine, but the heater radiator is not - i have checked the 2 pipes from under the dashboard, one is hot, one is not (i guess the return pipe is the cold one).

coolant level is fine...

clogged radiator? can i use some radiator additive flush to clean it? or do i have to use the method Dean mentioned?