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Discussion Starter #1
2004 Prius option 9 at 200K had hybrid battery replaced 1 year ago but this problem existed before that

Had AC compressor replaced as well a year go again this problem existed before that

OK so whenever you turn AC on regardless of the outside temp even when you first start car in the morning
the cooling fan runs constantly on and off. Appx 20 seconds on and 30 seconds off constantly even in the shade or a day with outside temp 65 degrees or even colder because I run checks on the AC even in the winter.

I've taken it to a hybrid specialist and they listened to it and don't think this is unusual.

I know because I spend a lot of time in my car that this problem did not always exist before. In fact I could set the temp to 73 in the shade with fan set to 2 bars on a hot daylike 90 and humid in the summer (southeast coast) and the ac would blow cold air and the car would be silent otherwise for long periods.

Code checks were done but I know codes may not register for a long time after you have been experiencing some condition for example the problem with the brake actuator would produce the sound for a long time before it actually gave a triangle and code and then I was able to get it replaced under extended warranty toyota had.

I suspect from doing searches online that it could be the thermal relay switch or whatever it is called or could it be the AC compressor itself running hot causing the fan to run on and off. Although it existed before the AC compressor was replaced is it possible the old AC compressor caused the problem and the new one was defective in some way to allow the problem to continue.

Maybe the same shop that replaced the compressor is brushing me off on this problem so they don't have to replace the AC compressor under warranty.

It is irritating hearing this fan going on and off constantly and I know it did not happen before and my gas mileage has taken a hit because of this I believe and the engine is running more frequently because the fan running too much and thus draining the battery thus affecting mileage.

When the car is driving not sure if this fan is running on and off as in park because of the road noise I can't hear it or is getting enough air flow.

I'm also thinking it's time to replace the 12V battery which hasn't been replaced in a while just as preventative maintenance as I have read in other posts people saying a new 12V battery increased their mpg because the hybrid battery wasn't being overworked by charging a failing battery.

I am going to do the battery check from the car screen.

I mention the 12V battery because I've heard reports of funny things happen in the prius with a failing 12V battery and wondering if the 12V battery may be indirectly contributing to this fan running on and off constantly.

I know the 04 Prius has dual fans one for the radiator and one for the ac condensor but don't know if they both come on at the same time or work independently but the fan closer to the drivers side is the one I can see turning when you lift the hood.

I know this isn't the hybrid battery fan which I just had inspected and cleaned because it rarely comes on because I keep the car parked in the shade and run the ac whenever it is hot and let windows down so hot air can escape.

I hope this is enough information but I will respond to all questions and be monitoring frequently.

Thank you in advance for any help.

I'm thinking of taking it to Toyota and have a hybrid cert mechanic listen to it with me present to get an second expert opinion and maybe have them check the AC compressor as it relates to the problem. Don't really like going to Toyota.

Alternatively just replace the 12V battery with an oem or that optima one which I planned to do anyways and see what happens.

Sometimes just replacing parts you have an informed educated guess that may be malfunctioning can be less expensive than letting shops rack up your bill doing expensive diagnostics beyond code checks.

Any thoughts?

Super Moderator
519 Posts
I think you have drawn a LOT of invalid conclusions here.

1)Anytime the AC is enabled, the compressor and the associated fans WILL run intermittently because part of the function is humidification.

2)A simple little fan will not draw enough power to have any measurable impact on your fuel mileage. (The compressor might.)

3) You might just be AWARE of what is happening.....that has been happening since day one. As things age, they tend to make more noise.

If this really bothers you so much, why don't you just turn the AC OFF ????
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