Hi there ,
Had the same problem on my IS300 H 2014'. Followed all of your comments i managed to get is sorted out as well. It was a bit tricky to find , but it turned up well.
The fan is located in the boot, on the right hand side above the wheel arch.
Firstly you need to plug out the plastic pins that hold in the material onto the bodywork ( no need to remove it completely , just to have accesss to the fan ) , after you get access to the fan , remove it from the bolts that hold it in on the bodywork ( should be 3 long screws ) ADVICE : Before performing this , disconnect the car battery so no current flows through the circuits. Also , do not unplug the air fan from it's wires !! Just simply unscrew the bolts that hold it together and the air fan should come out into two pieces, that way you do not have to take it out completely and the access is good.
For the ones thinking it's a big job or need to consult a dealer, absolutely no need. You indeed need to be careful on what you do , but with gentle care and no wires disconnected , you should be fine. The dirt from the fan could be easily removed with a new soft paint brush if you do it properly and gently from the outside of the fan radius , press a bit harder first , the dust is stuck properly to the vents - it will get a bit dusty in the boot by doing this , so get a cloth and cover the other intake from the flat black plastic tube that goes down underneath the cover towards the batteries , you do not want dirt to get in there.
I will upload some picz i took so you lot kinda see what i done if you are confused.
P.S - clean the dust on the air intake for the fan as well , located on the right hand side from the back seats between the seat and the door.
The warning message disappeared instantly after plugged the battery back in and took it for a spin.
Wish you all luck 🙂
Pictures were made after cleaning the fan ( it was nearly black and dusty )