Ticking sound

There are three motors behind the dash that control the fresh air/recirculation mix, the hot and cold air mix, and the vents from where the air will enter the cabin.

It sounds like one of these motors cannot find its position and keeps seeking.

If it is one of these motors, then it will have set an air conditioner code in the body ECU. The is a key press sequence using the steering wheel controls that will display the code in the top right on the MFD. You can get the codes by turning the car on while holding the AUTO and FRESH/RECIRC buttons. If you get any codes (other than 21), you can report them here for further assistance.

 

In my experience, hunting of the HVAC servos can take place without always incurring trouble codes.

What generally happens is the position sensor on the servo, because its contacts are dirty/oxidized, sends a fluctuating signal back to the controlling ECU. So the ECU thinks the door has moved from its target position, and controls the motor to move it "back". Which of course changes the position signal again, and the ECU controls the motor to move it "back". And so on.

The ECU doesn't necessarily notice this is a problem and set a trouble code. It has codes for if the position sense signal goes outside the range of normal, and codes for if the motor is being powered and the position reading isn't changing at all. But if all the ECU thinks is happening is it's correcting the position in response to the position changing, well, that's what it's built to do, so doesn't necessarily raise a code.

Here's the thread about it, with a video:

No, that wasn't a mouse in the heater | PriusChat

 

If changing the vent openings on the HVAC settings doesn't have any effect on the sound then it's likely the 3-way coolant valve and you'll soon get a check engine light with error code P1121 stored.

 

The topic of the thread is the same.

There are three different servo motors. One is for the "inlet door", which controls whether the HVAC takes air in from outside (fresh) or inside (recirculate). One is for the "outlet door", which controls which outlet (feet, face, windshield, etc.) the air will blow out from. And one is for the "blend door", which controls the temperature of the outlet air, by varying the amount of it that goes through the heater core versus going around it. (Another name for that one is "air mix door". And to keep things confusing, a lot of people for some reason use the name "blend door" no matter which door they're really talking about.)

If I remember right, the one shown in the video in the not-a-mouse-thread happens to be the actual blend door. The outlet servo in that generation is near it, on the same side of the heater box, and not much harder to access. The inlet door is over on the other side between the heater and blower boxes and a PITA to access.

Whether you notice a difference when you change HVAC controls depends a bit on which of the three servos happens to be not-a-mousing.


The coolant valve out under the hood can exhibit the same kind of problem, but there are some other easy ways to distinguish that from an HVAC servo.

The HVAC servo linkages move more lightly, which is why they sound kind of like a mouse scrabbling about, and of course are louder inside the cabin than under the hood.

The coolant control is a water valve with a higher-torque servo to move it, and when hunting back and forth it sounds less like a mouse, more like a washing machine or an old floppy-disk drive. And it, naturally, is louder out under the hood.

 

Sounds like the air outlet servo. Not exactly the one shown in the not-a-mouse video, but the one next to it, a bit more difficult to access, but still nothing like the one that's hidden over on the other side.

For gen 2, be sure to read along in the not-a-mouse thread to see the tool yitznewton used to get to the screws in a gen 2. Access was easier in gen 1, but gen 2 happened to stick the brake ECU down there right where it kinda blocks access.