2012 with 279k miles overheating

B1503 code is pretty specific. Maybe look ➡here⬅.

 

B1503 Exhaust Heat Recirclation System overheating after sealant:


This is pointing to a) no flow through the heater (there is no valve in the heater loop), b) heater core restrictions caused by the sealer, c) clogged engine thermostat d) reduced flow from the engine water pump (typically a cracked or swollen pump impeller) or e) insufficient coolant volume resulting in the heater core being air locked (a hg side effect).

Radiator clogging is not common and aftermarket radiators or water pumps are not recommended.

Its not clear how you verified the water pump flow but it can be intermittent or it can have partial flow without setting its own code.

However your b1503 code directly points to the Exhaust Heat Recirculation System (EHRS) which has a second coolant temperature sensor downstream of that system on a small diameter hose.



That sensor sets your code at 248f, which is over 50f hotter than normal operating temperatures. Not good.

Monitoring both temperature sensors helps by using Toyota Techstream or a good bluetooth based app like "Car Scanner". Car Scanner is easier to read and graph. Using Techstream, you can read one of them in the Data List for "Engine and ECT", and the other one in the Data List for "Combination Meter".



Normally exhaust flow through the EHRS system is diverted through its heat exchanger when cold and mechanically opens when hot to allow straight through flow.



We have seen cases where the EHRS remains closed in its initial diverted state causing high downstream temperatures which then trigger the b1503 sensor.

A few stuck EHRS cases without other factors like sealer, head gasket or water pump issues have not caused overheating on the engine block as reported by the other temperature sensor.

Further isolation is needed. This alone can cause 248f throughout the system triggering the one code you read before the system goes to a limp mode. The fact your reservoir cap relieves indicates high temperature and pressure there although the reservoir loop bypasses radiator cooling.

However water pump or EHRS system problems may exist. Careful live data temperature monitoring can help.

 

Any block sealant can and will block any narrow passages; including and not limited to the thermostat, radiator, EGHE, heater core, and coolant switch valve - if so equipped. I'd pull the bottom radiator hose to see if there's any sludge there. I believe that stuff is heat activated, so that stuff is more than likely solidified in the EGHE, as well as other areas of your coolant loops....

 

Stop-leak should be illegal.

Of interest, asking Google AI about how to dissolve/flush stop-leak products:

https://share.google/aimode/KLM19femvijjfffPa

And specifics about what cleaner to use:

https://share.google/aimode/O1p19gwBdiy8NOEBd