Heater blasts hot air on "HI", but the heat stops with one click to a lower temperature. Blower work

And you've had no temperature light pop up on your dash? And after this hour of driving if you were to lift the hood and touch related rubber hoses the ones that are an inch or better and they are all reasonably warm and similar temp? If yes I would say maybe it's the electronic dash portion that allows you to do this temperature setting by temperature they used to go bad and Cadillacs and other vehicles like that all the time. Maybe it even has its own heat ECU..

 

That's nearly pathognomonic of a problem in the wiring to the cabin temperature sensor.

The temperature settings you can select are LO (blast me cooling no matter what), HI (blast me heat no matter what), and all the numbers in between, where it looks at the cabin temperature sensor to see if cooling or heating is needed.

So if the temperature sensor is giving a bogus high reading, then HI will still blast you heat no matter what, but no actual number setting will give you any heat.

The other way that can go is if the cabin sensor gives a bogus low reading; then the LO setting will still blast you cooling no matter what, but no actual number setting will give you any cooling.

Toyota tends to use negative-temperature-coefficient thermistors for these sensors: the resistance goes down as the temperature goes up:



So a bogus high reading from the sensor would mean the resistance is too low, as would happen if the wires going to the sensor got damaged and shorted.

(If the wires got damaged and broken, or someone did work in the dash and left the sensor unplugged, the resistance would be too high, it would look like a low temperature, and you'd have the "only cools on LO" situation).

If you had a scan tool you could connect and look at the a/c system data list, you would probably see a fixed, bogus high reading for the cabin temperature, and that would confirm this is the problem. But this is that kind of problem where the symptoms you've already mentioned pretty much nail it.

You might have an a/c system trouble code B1411, which is supposed to tell you about an open circuit or a short going to that sensor. If you hold down the AUTO and FRESH/RECIRC buttons while powering the car on, you might see code 11 in the a/c display. Or you might have an issue where the wiring isn't completely shorted, so it gives a reading that is bogus high but not as high as the 135℉ a true dead short would look like, and doesn't quite trigger the code.

In any case, you can find troubleshooting workup steps you can follow if you look up the B1411 troubleshooting section in the repair manual.

Toyota Service Information and Where To Find It | PriusChat

 

Also check where that sensor is the little vents right below the key slot or wherever it is in your car Make sure that's not completely plugged with dust bunnies or something a piece of tape over it I don't know

 

The actuator would be a more likely suspect if you either had heat all the time whether you want it or not, or no heat all the time whether you want it or not.

If the actuator is able to zip right to full heat when you request HI (heat no matter what), but then zips right back to no heat as soon as you ask for any controlled temperature, then that tends to confirm the actuator works, and the problem is with what the system thinks the cabin temperature is.

 

I'll bet it turns out even simpler than that. Thermistors are too simple to go bad much. My money's on a bit of damage to a wire to the thermistor.

 

"followed the Toyota diagnostic steps" ?!? Yeah, for some other problem!

The only "illumination rate" PID that I know about is the one in the Body ECU that tells you the reading from the solar sensor on the dash that is used for the auto headlight feature.

Did you say, by any chance, anything to them about a problem with the auto headlights? Does your Gen 3 even have the auto headlights?

The same sensor on the dash that supplies the "illumination rate" to the Body ECU also supplies a "Solar sensor" PID that can be read from the A/C ECU. (It comes out on separate wires; that sensor is really two distinct sensors. I don't know the details, but the one for the auto headlights might be tuned more for visible light and the one for A/C more for radiant heat.) The A/C ECU "solar sensor" PID will be shown as a value 0 through 255.

Neither of those has anything at all to do with the room temperature sensor. For that, they should be looking at the "room temperature sensor" PID from the A/C ECU.

It's hard to even guess what problem they thought they were diagnosing.

 

The ECU that controls the HVAC has an odd name: it's called the "A/C amplifier" for historical reasons. In a modern car it's just an ECU like any other. It lives below the HVAC unit, near the firewall, just above the center hump in the floor.

So the room temperature sensor is wired to that ECU. That ECU reads the temperature (and will happily tell you what temperature it reads, if you plug in a scan tool), and that ECU decides what to tell the blend (also known as air mix) actuator to do.

There is no "blender actuator temperature". There are two PIDs called "Air Mix Servo Targ Pulse" and "Air Mix Servo Actual Pulse" and they give the position of the door, counted in pulses. The target pulse is where the ECU wants the door to go, and the actual pulse is where the door currently is.

The purpose of the door is to direct air flowing through the box, so that it either all goes through the heater core (and gets heated), or it all goes around the heater core (and doesn't get heated), or goes partly through the core and partly around it (and gets partly heated).

The door positions range from pulse 6 (all air going around the heater core, adding no heat) to pulse 93 (all air going through the heater core, to pick up maximum heat).

If the ECU thinks you want a temperature that's below the current room temperature, it's perfectly normal for the ECU to send that door all the way to position 6. No point adding any heat at all, if you want the cabin cooler.

Likewise, if the ECU thinks you want a temperature that's above the current room temperature, it's perfectly normal for the ECU to send the door straight to position 93. It will give you all the heat that it can, until the room temperature starts getting close to what you've set.

Only when the room temperature is getting pretty close to your setpoint will you start to see the ECU sending the blend door to positions in between 6 and 93, as it tries to fine tune just the amount of heating needed to stay at your setpoint. You can think of the blend door position as a very S-shaped curve: whenever the actual room temperature is more than slightly above or below your setpoint, the door will be at one extreme or the other, and it only modulates to middle positions when the room temperature is very close to your setpoint.

So, as already kind of covered in #6 and #9, all the behavior they're seeing is completely consistent with the room temperature reading appearing to be higher than 85 ℉. When you select HI the ECU says "ok, the human wants max heat no matter what, I'm sending the blend door to position 93." As soon as you select one notch down to 85 ℉, the ECU says "whoops, the cabin is hotter than the human wants, I'm sending the blend door to position 6."

It is all behaving exactly as it would be supposed to behave, if the cabin temperature really were above 85 ℉.

If the techs at this dealer were surprised by the blend door behaving exactly the way it would normally behave in that situation, it's possible they just aren't the right people to solve this problem.

Edit: the specific pulse counts 6 and 93 I'm using here are from the Gen 3 liftback manual. Details like that could be a little different in a v, without changing the basic way the system works.

 

Ah, so you do have the auto headlights. Yes, that would explain why they'd have been talking about illumination rate. That reading comes from the little dome sensor in the dash under the windshield. The same little dome also has a sensor wired to the "A/C amplifier".

If there were rodent teeth involved, lots of things could have happened. Another thing that could have happened would have been techs checking out that sensor and leaving something unplugged.

I'm still not sure why they got interested in the solar sensor or the illumination rate in the first place, when you showed up with a question about the room temperature sensor.