No cabin heat. Overheat icon flashing.

There is another coolant sensor on the exhaust gas heat exchanger circuit, but since you have that bypassed - that shouldn't trigger an over-temp event. The gas flapper valve would get stuck closed, causing extra hot coolant to enter the cooling system - the over-temp lamp would trigger, but if you don't use a scan tool, you wouldn't know that the exhaust gas coolant sensor was triggering the over-temp lamp; NOT that the actual engine was over-temping.

 

If the OP is no longer getting overheat warnings, I'd suggest looking into the hvac control. I had a similar issue with a c-max energi about a decade ago but it would only happen in hybrid mode and only on the passenger side. A new blend door controller fixed it. There were no codes or icons flashing.

The OP's overheat issue could have come from a botched fix attempt (air in system) by the PO chasing the wrong fix solution for no hot air.

If it's not too cold out, the OP can try to keep it in ev mode and see if the electric heat system functions.

YMMV

 

I recall someone else found something similar; you may want to do a search.

 

Engine cooling has its own path from the engine to the radiator and back to the engine. The other flows could be blocked without impacting the engine.


However the biggest clue is you get cabin heat after a coolant flush. Cabin heat stops shortly after. That normally (>95%) means you are quickly losing coolant and the first indicator is a loss of cabin heat. The second indicator is a flash or constant overheat light (at a rather high 248f unless the water pump is also coding).

A common gen4 coolant leak is the exhaust heat recovery/recirculation system through its internal heat exchanger into the exhaust pipe.

A common diagnostic and fix is to bypass the exhaust heat recovery system. There are two methods; no parts required at the engine or the one extra hose at the exhaust heat exchanger. There are many posts on these techniques.

Toyota acknowledgment of the problem:

Gen4s don't have the "no overheat/ no good reason" head gasket fails that gen3s are known for but they can still blow head gaskets with an overheat.

This is the second sensor. The first is on the engine. As stated before you need to monitor both with a scanner. In Techstream and most scanners, both are called Engine Coolant Temperature but originate from different ecus.

Sounds like stop leak. Hopefully not. Sometimes when an aluminum engine overheats it blows the head gasket. It is a well know car flipper trick to use "block sealer/coolant stop leak" to temporarily seal the head gasket leak. Temporary can be 1-52 weeks.

Some residual sealer and bypass pics