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Defrost Fans Providing Sub-Optimal Output

Hi there,
I own a 2017 6 sport, 50k mi, only one incident of collision damage while I was parked on a side street and it was all surface-level. I honestly can't remember when I started noticing this problem, but I don't recall it ever being as bad as it has been so far this winter. Essentially, the defrost fans are working at around 50-70% efficiency. I run them with the A/C on to reduce humidity, full heat, no circulation. The front windshield stays mostly defrosted, but about 2-3 inches running down both sides and across the top stay frozen, enough for my wipers to scrape against chunks of re-frozen ice that's been melted and blown back from the center of the glass. The front passenger windows only defrost about halfway back, and the rear passenger windows get completely frozen over to the point where I can't see out of them at all. This is obviously a major safety hazard. I have no idea what could be going wrong. Bad blower motor maybe?

Any advice would be appreciated. Thanks!
 
In the meantime, my solution has been to crack both rear windows about an inch, and that seems to be getting me to work safely until I can make it to a dealership to inquire about it. I'll share here if I learn anything.
 
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2020 CX-5 AWD
…. I run them with the A/C on to reduce humidity...
I'm not familiar with your vehicle, but most newer systems run the A/C automatically and unconditionally when the defrost setting is selected. However, you should never need (or want) to run A/C with a non-defrost heat setting (if that's what you're doing).

With the vehicle completely warmed up (i.e. run for 15 minutes), if you turn the heat up all the way, with the A/C off, and the fan on a medium setting, how hot is the heat output at a middle vent? Uncomfortably hot, just warm, or not even that? Also, is there much change in heat output temp at that vent, as you change blower settings?

My hands-off guess would be that your vehicle has some type of heat output/delivery issue and not a blower problem. But if you answer the questions, perhaps things might become a bit clearer.
 
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I'll give that experiment a shot and report back. I know the other evening after work it was very cold, somewhere around 5 degrees F, and my girlfriend and I sat idling for maybe 10 minutes before deciding it wasn't gonna do anything more than defrost maybe the inner 50% of the windshield until I started driving the car. The air was perhaps mildly warm at the vent, certainly wasn't warming the cabin. After another 5-10 minutes of driving the car, it got up to temperature, but I still had issues defrosting the full windshield and the rear passenger windows.
 
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Hmm, nope, I do. I'm pretty nitpicky about getting corner to corner. This also happens regardless of whether I start off with ice on the car.
 
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2020 CX-5 AWD
Hmm, nope, I do. I'm pretty nitpicky about getting corner to corner. ..
Ok on that - not sitting in the vehicle with you, I'm just trying to piece everything together based on what you've written. And I definitely don't have a complete picture yet. So if you want to bear with me a bit, maybe I can get a better idea what's going on. Rather than asking you to describe exactly what you do before putting the vehicle on the road, I'll tell you what I've done for the past 50+ years of driving. This is not intended to be a lecture here on what you should be doing, just what has always worked for me with every vehicle I've ever owned. And then perhaps we can find out if something that you're doing could be changed a bit, to avoid what's been happening to you.

Before starting the vehicle, (if necessary) I first scrape off the outside of all the windows, 'corner to corner' as you wrote above. So at this point all of the windows are completely free of any coating, inside or out. Then I start the vehicle and warm it up at idle for around 1 minute (or 2 on a day around zero F), with the blower not running at all. Then I start travelling on the road, with the blower still shut down and the setting not on defrost. At this point, with the cabin and outside temps not greatly different, there will still be nothing on the windows, inside or out. Now after another 5 minutes or so (which varies by vehicle), the heat will have become adequate, and I turn the blower to a low/medium setting - just one of the vent selections, no defrost because the windshield still does not be get coated.

This routine of course applies to clear days only, and certain types of precipitation will often require the blower and defrost to be used early on to keep the windshield clear. So we're talking about clear days only right now.

Now I know that most people don't want to put up with a vehicle being cold for this long, which is the reason remote starts have become so popular. So it sounds like you're trying to get the vehicle warm inside before starting out, but correct me if that's not the case.

If you have never tried the routine I described above, and are up for experimenting a bit, then give it a try one time. If it doesn't keep the windows completely clean, then something is definitely wrong with your HVAC system, because this should always work as I described. But assuming it does work, then you could try tweaking the details, and maybe come up with something that works well for you.

I'm not saying there cannot be something wrong with your system - just trying to find a way to discuss it without being able to sit in your vehicle.
 
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So I did the experiment you suggested the other day after getting off from work last night. After 15 minutes of idling, the engine was only at about 90F, and the middle defrost vent was only putting out mildly warm air. Thankfully it wasn't too cold last night, about 26F, so the moisture buildup on the rear windows was minimal, just some bordering the corners of each pane. However, the strangest thing happened after I started driving. Within 10 minutes, the engine was fully up to temp, around 170F, and the air coming out of the middle vent was now uncomfortably hot, but the rear passenger windows had actually gotten worse. Which makes me believe it is an airflow issue, and the increased moisture buildup on the inside of the rear windows is being perpetuated every time I drive by the cold air blowing past the window and keeping it cool.

The suggestion you made in your last post is very reminiscent of what I remember my parents doing when I was a kid. I will say I never try to bring the car "up to temp" before driving off, unless it's something wild like -10F, in which case I'll usually try and do it for the sake of the car. But most every time, I hop in, turn it on, turn the defrost on medium or high so that it starts working as soon as possible, and drive off. Usually within 5-10 minutes of driving the air starts to feel warm, and then within another 5 minutes it gets almost unbearably hot on the highest temp setting, but again, the rear windows still remain foggy, especially on days below 20F. The only time I had actual ice form on the inside of the window was a couple weeks ago when it was 5F after sundown and I'd just been driving with the defrost on full heat/power for about 20 minutes.
 
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2020 CX-5 AWD
So I did the experiment you suggested the other day after getting off from work last night. After 15 minutes of idling, the engine was only at about 90F, …. Within 10 minutes, the engine was fully up to temp, around 170F....
Well it's really helpful that you made that observation and posted it back here, because IMO that sounds highly likely to be the thermostat sticking open. Mazda has released a TSB describing this issue for the 18-20 Mazda6, so it seems reasonable to assume this might apply to some 2017 vehicles as well.

And if this problem does turn out to be a bad thermostat, then I will issue a retraction on everything I've written about possibly changing your warm-up routine, because there would be nothing you can do to prevent this stuff from happening🙃
 
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That would make sense, I agree. However, the strangest part that (as far as I know) remains unexplained is that even once the air coming from my defrost vents at full blast is uncomfortably hot, the rear windows still fog up on cold days. I feel like that would be a blower issue. But again, I can't be certain. I'll have to take it in, I suppose. I can't seem to find any other posts about this issue online that have been publicly resolved. Thanks for all your assistance!
 
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2017 Mazda 6 Sport
That would make sense, I agree. However, the strangest part that (as far as I know) remains unexplained is that even once the air coming from my defrost vents at full blast is uncomfortably hot, the rear windows still fog up on cold days. I feel like that would be a blower issue. But again, I can't be certain. I'll have to take it in, I suppose. I can't seem to find any other posts about this issue online that have been publicly resolved. Thanks for all your assistance!
There are no defrost vents for the rear windows. They are only under the front windshield and the small ones that point at the front passenger windows. The rear windshield as the rear defrost/defog heating element of course
 
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There are no defrost vents for the rear windows. They are only under the front windshield and the small ones that point at the front passenger windows. The rear windshield as the rear defrost/defog heating element of course
I'm aware. But regardless of where my fans are located, if everything is working as intended, my rear passenger windows should not be covered with fog or ice, creating blind spots while I'm driving.
 
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‘17 CX9 & ‘19 3 GT
I'm aware. But regardless of where my fans are located, if everything is working as intended, my rear passenger windows should not be covered with fog or ice, creating blind spots while I'm driving.
Keep your windshields and windows clean on the inside! Dirt, moisture and humidity in cabin causes moisture build up on the windows especially on cold days! Also ensure that your floor mats are dry as snow turns to water and water turns to moisture and humidity. Also note that the Mazda vehicles‘ climate control doesn’t heat up fully (even if set to highest temperature) when idling. When you start to move only then will it engage to the full climate setting...

If your car has auto climate settings, I suggest when initially defrosting the windshield & windows to turn AC off and manual climate settings then once defrost, switch it to auto setting which will turn the AC on automatically. The AC regulates the humidity levels in the cabin regardless of the season. Hope this helps...
 
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