Delayed cabin heat and red triangle accompanied by a thermostat symbol

i would start by having the codes read, or getting mini vci if you plan on doing work yourself.

 

Here are some possibles along with a stuck thermostate that is OPEN which would be too much flow and not controlled.
P1123 2005 TOYOTA PRIUS Coolant Flow Control Valve Position Sensor Circuit High | AutoCodes

 

I'm going to guess there's something wrong with the coolant level or the heater circulating pump (there's an electric pump for the heater core, not the engine coolant pump). I think the warning lights you're getting are for high temps, not low temps. I would definitely not block air flow at the grille.

Have you checked coolant level in the COLD radiator (not the plastic reservoir--you need to pull the black cover over the radiator cap). If there's air in the system, it's very difficult to bleed on this car. There are other threads here about that.

 

No, you cannot make that assumption. In fact the most likely cause of your symptoms is that you lack sufficient coolant in the radiator and/or have air in the cabin heater core. That is causing the lack of cabin heat and the engine overheating warning.

See if your engine coolant pump is leaking coolant out of the pump shaft bearing. Typically this is manifested by a pinkish stain upon the AC compressor housing, on the inside of the pump pulley, possibly sprayed on the underhood sound insulation pad, etc. If so, replace the coolant pump.

The only thing special is that you need to do a very good job getting air out of the engine coolant loop. I have posted on how to do that and on how to run the coolant heat recovery system pump manually to fill the CHRS canister, if you do not have Mini VCI to run the pump via software command.

 

I was once driving up through Chicagoland in a car borrowed from a friend, which turned out to lose coolant. Got through town finally, up by the botanical gardens, glanced down at the dash and saw the temp gauge nearly pegged hot.

So I reached down immediately and turned the heat up full blast (in August...) just to extract some more heat that way, and ... no heat came out. Bad sign ... too little coolant to even circulate to the cabin. Next move was quick exit and pull off the road....

-Chap

 

The next worse thing is that your gauge starts to go to cooler meaning that coolant is no longer being read but only HOT vapor is passing by the sensor which it cannot read.. Next Stop Blown Head gaskets!

 

Did you ever discover the source of this problem? I have the exact symptoms and have already replaced the coolant pump after it threw the code indicating there was an issue with it. When it was pulled there were stains from a leak.