I am sharing some info in case it helps anyone in the future. I am in the DC Metro area and have a 2018 RX350 F Sport with ~74k miles. This winter while driving around in snowy conditions with a lot of salt and snow on the road, I got the ICS malfunction warning, which throws a CEL, puts it in 2WD mode, and disables cruise control. Most online guidance says to clean the sensors (snow and salt blocks them) and disconnect the battery to reset it. I did this and then had the same issue about 200 miles later, again while driving around in snow and salt. After the second issue, I bought a LAUNCH CReader Elite V2.0 (works really well) to check on the CEL and found a few faults:
P0128 (thermostat stuck open, which is a DTC / ECM code)
C1280 (AWD malfunction)
C1A40 (powertrain sys malfunction)
B2312 (window motor malfunction)
C13AF (parking break malfunction)
C13A6 (parking break malfunction)

I cared very little about the issues for the thermostat, window motor, and parking break, but after much online reading learned that the most likely cause for the ICS malfunction is the thermostat. The theory is that the thermostat (P0128) is the root cause of three ICS/AWD codes. The 4WD ECU outputs C1280 when the ECM circuit malfunctions. Communication is technically normal, but the engine control system is reported as malfunctioning. Similarly, C1A40 is stored when the EFI system malfunction signal sent by the ECM is detected by the clearance warning ECU assembly. All of these are safety-based malfunctions that disable safety-critical features (autobraking, radar cruise control, etc). The theory is if you ever see C1A40 or C1280 appear together with an ECM fault code, always treat them as dependent symptoms, not independent problems.

I did notice that the temperature gauge only topped out a quarter of the way up, and my MPG had dropped ~2 mpg, confirming the faulty thermostat. I didn't find much online suggesting the thermostat was a common issue, so I guess I'm a little bit of an outlier there. I called my Lexus dealership to learn they will charge $2200+ for the thermostat replacement. My local indie shop warned me that it was a tough job because the engine sometimes needs to be dropped. They wouldn't quote me without bringing it in and paying a diagnostic fee that would be applied to the repair cost. There wasn't much online help regarding repair instructions, so I figured I would give it a try. A couple of threads out there says the engine doesn't actually need to be dropped, including one where a service tech says they don't drop the engine. I wasn't encouraged by the lack of DIY material, but I also wasn't discouraged enough. Spoiler, I don't have much to offer there and was not successful.

I purchased 3 bottles of Toyota coolant (00272-SLLC2), a thermostat assembly (16031-31031), replacement drive belt (90916-A2026), and a no-spill funnel set (Lisle 24680 (MSCLIS24680)) and gave it my best shot. I was able to drain the cooling system, including from the aft engine drain plug (the plug is directly below where the exhaust manifold bolts to the block), and change the drive belt. I removed as many parts from the top of the engine bay as I could, including the entire washer platform, the ECU, and even the brake controlling unit (to gently push it to the side), but the engine mount is still very much directly in the way. I then went from underneath and was able to remove the passenger wheel and plastic engine bay access panel, the drive belt, and the four bolts from the belt pulley on the water pump. I couldn't get the pulley to fully remove from the water pump because of a lack of clearance (frame), but I could push it out of the way enough to get to the three bolts on the thermostat and the thermostat's electrical connector. The insurmountable issue was that the aft thermostat hose was connected with a spring clamp, and the clamp pieces that needed to be squeezed were on the top side of the hose and not accessible from below. I could not get access to the clamp from the top or the bottom or even twist it around to grab from the bottom. The only other solution I could think of was to leave that hose connected to the thermostat and remove it and the thermostat at the same time. I hadn't purchased that hose though, so it wasn't an option. So I put everything back together again without replacing the thermostat housing. The no-spill funnel was excellent for refilling and bleeding the system (no purge valve on the 2018). At least I installed the new drive belt. I ended up taking it to my local Toyota dealership today and paid $1150 for them to drop the engine enough to get at the thermostat. Just sharing a couple of data points for the repair cost (twice as expensive at Lexus vs Toyota) and the link between the P0128 code and the ICS malfunction warning. I'll update if there are any other issues over the next few weeks. Hope this helps someone in the future.

 

so at the end of the day, we can't really do it yourself to replace the thermostat? unless we drop the engine?

 

I was not able to. Maybe someone else has been successful and will be willing to post some tips. I couldn't find instructions on how to drop the engine either.

 

Yes you can diy. OP was close just needed to remove upper motor mount and change it from the top. I did mine along with the water pump. Tutorial can be found on youtube.

 

Check this video out. After you remove the engine mount and the PCM it will become accessible.

 

That's a great video. I wish I thought to search for water pump in my research. Probably should have paid a few $100 more to have the dealer swap the water pump while they were at it.