Need help diagnosing my 2016 Prius

Search this site for heat exchanger

 

You're losing coolant through the heat exchanger in the exhaust pipe. There's a work-around that involves bypassing the heat exchanger circuit with a U-shaped coolant hose. The other option is replacing the portion of the exhaust pipe with the heat exchanger.

 

Those are incentives for solving this problem as quickly as possible, and not continuing to drive unnecessarily.

2. The "coolant light" isn't a warning about the coolant, it's about the engine being too hot right now. The right time for a coolant warning would be earlier. The light coming on is bad.

4. The coolant temperature sensor is a "negative-temperature-coefficient thermistor", which means the input goes lower for higher temperatures. When you get the code about the input being so low the ECM thinks it's a sensor or wiring problem, in this case it's the engine getting so hot the ECM doubts that could even be a real reading. That's hot....

 

There's a TSB on the 2016 exhaust gas heat exchanger and may be covered under state or federal emissions warranty. If the car has a salvage title all bets are off.... That's assuming you haven't blow or burnt up the engine by continuously running it HOT.

Hope this helps....

 

For reference, PIPE ASSY, EXHAUST, FRONT:

Exhaust pipe for Toyota Prius XW50, 4 generation 09.2015 - 11.2018 - Toyota Car and Auto Spare Parts - Genuine Online Car Parts Catalogue - Amayama

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If the coolant is not entering the exhaust via the heat exchanger, then the only other way is if there is a blown head gasket, Blown Head Gasket Symptoms & Causes | Fel-Pro Gaskets