So I just bought a 2007 Prius with 345k miles on it. It's in great shape, good looking little car, great on gas, no major issues. New hybrid battery and 12v within the last year. The problem is that regardless of outside temp, regardless of inside temp, if I turn the climate control on, and set LO temp, it blows wide open cold. But if I turn it up one click to 65 temp, it flips over to wide open hot and never comes down from that. I suspect a temp sensing issue because if I turn the auto function on, the temperature does the same thing, but in both temperature settings it blasts the fan as if trying to catch up. I'm mechanically inclined, but I'm new to these cars and I believe in asking advice from people who have more experience. Thanks in advance!
The usual cause of that behavior is exactly what you have guessed. When you set for LO, you get wide open cold no matter what, but one click up and it needs the input from the cabin temperature sensor. If that sensor is giving a false reading lower than 65, then the system will always be trying to heat. That sensor is behind a little panel low down on the dash, above your right knee. In some generations there are some louvers on that panel, but I don't remember yes or no for Gen 2 for sure. It is an electrical sensor, with wires plugged onto it, but there is also a small hose connected to it, running over to the HVAC blower housing at the right of the dash. That uses suction from the blower to draw air from the cabin over the sensor, so it isn't just reading temperature of some still air above your knee. If you have Techstream, or some other diagnostic tool that can read live data, you can see what temperature the ECU thinks that sensor is reading. Even without Techstream, if you hold the AUTO and FRESH/RECIRC buttons (on the wheel) while powering the car on, the HVAC will do some self tests and may show some two-digit trouble codes at the top of the MFD screen. That might be worth doing.
That is a great tip, @ChapmanF Is there a reference somewhere for other similar diagnostic triggers aka,“Easter Eggs” for other things on the Prius ?
