Heater is constantly on! Help!! Summer is coming!

It sounds like the heat pump is working correctly when you're on electric power, but when the engine is running its hot coolant is providing heat through a failed valve or hot air damper. Both are normal sources of heat in certain circumstances. Is your car still in the original warranty time & mileage, or is it beyond that. Maybe try an independent shop that works on Japanese cars.

 

Prii have historically used a hot air damper for this, not a coolant valve. I don't have a Prime right here to look at but I assume that hasn't changed.

This seems kind of early for the "air mix damper servo" to begin acting up, but such things do happen, and produce the exact symptoms described.

 

Yours is not a common problem. Anything can go wrong any time, and you have a problem new to us. Please let us know what part failed.

 

The air mix damper failure comes up often enough on older ones, but it's pretty rare on one as young as yours.

Lots of other brands use the same type of system, and they fail the same way. I dealt with it in one of my Subarus years ago.

 

Looking for "air mix damper failure" on Google brings Lexus and Toyota as the main hits for "air mix damper servo failure" and it says it returns a ECU code. I hope that damper for the Prius isn't under the dash because it does sound expensive, needing the whole dash to be removed on a Lexus

 

A manual bypass of the coolant lines going to the heater core in the engine bay might be easier than dash work.

 

That seems a little drastic. When you wanted heat you would have to stop, open the hood, open the valve……, just saying.

 

It depends on how the HVAC box is built and just where the damper servos ended up. In gen 2 at least a couple of them were on the easy-to-get-to side (including the air-mix one). Gen 3 managed to put all three of them on the it's-a-headache side. Thanks Toyota!

I don't know about the Prime. Look at the diagram, see how lucky you'll be:

Toyota Service Information and Where To Find It | PriusChat

There are different kinds of 'failure'. Sometimes something electrical goes wrong with the servo. Sometimes something mechanical goes wrong with the flap inside the box. Just have to see what happened in your own case.

Even when the repair manual shows the whole dash coming out, sometimes people find ways. I read a post here on PriusChat once where somebody found their damper had popped loose from the gearing, inside the box, which sounded like the worst case for getting repair access to, but they managed to pop it back into place using, like, an inspection camera and chopsticks or something poked up through a vent, like laparoscopic surgery.

 

It's a Prime with the heat pump. Should only need to switch the valve for winter's beginning and end. I do admit it is drastic, but an option depending on what it takes to do a proper repair.