Turn your cabin heat on full hot, front and rear heat. Fan on high. Does heat blow out vents: very very hot, lukewarm or cool. If not very hot. Likely coolant level is low.
I would first, check coolant level.
Park vehicle overnight (!8hrs), after warming up engine and bring RPM up to 3K few minutes.
Park on level ground. Even better. With front end higher than rear. This insures, top of radiator is high point of coolant system. Air travel to high point.
In the early AM, just before sun-up. When the very coolest OAT. Before sun warms the air. Check under radiator cap. Fill radiator, to the very top. Fill reservoir (expansion tank) to high line.
If you find low, repeat each morning. Keep repeating until not found low. May take 3 days.
Thermostat do fail. Some, where they get stuck open or opens at too lower a temperature. This also, can result in low engine coolant temp (ECT).
In some cases, a new thermostat, is installed wrong. Toyota thermostat, has jiggle valve. Jiggle valve must be at top. Jiggle valve, is parts of how system bleeds air.
I've a 06LC w/192K I just bought. In reading history I found PO complained of engine temp raising when both front and rear AC on. Toyota Dealership diagnosed with clogged radiator fins. Recommended new OEM radiator, $600 for the US spec Radaitor plus coolant and labor. They should have added radiator cap and thermostat to job, as best practice.
Got me to thinking about the radiators I've been replacing. All have debris built up on lower third.
This one hand some extra grasses along with general clogging on lower third.
Over the years/miles debit builds up in condenser and...
