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According to the manual for my 2013 Honda Fit, the button for the rear windshield defoggers also heat the side mirrors. Is there any way to confirm that the side mirror heating is actually working? I can't feel anything when I put my hand on it. Should one be able to feel the warmth of the heating on the mirrors?

The background to this situation is that my car was parked in a 3rd level sub-basement last year, and it was flooded. The remediation took so long that it was spring by the time I got my car back. Part of the remediation was to replace the electrical wires and modules. A few things got overlooked, e.g., reconnection of the trunk lights. When winter came back this year, I thought that the rear defogger wasn't working and Honda did tests to show that it did. It turned out that the defoggers just aren't that effective on the Fit, taking a long time to heat the windshield. It took a lot of effort to have Honda check it, however, and I just didn't have to energy to bug them more about checking the side mirror heating (even though I did mention my concern about it when I complained about the rear defogger). As you might imagine, I have no desire to impose on them further to check the side mirrors if there is a way for me to check them myself.

It's hard to test because I often brush off the snow before driving off, for the sake of safety. As for not brushing off the snow and keeping the car mostly stationary, the manual advises against that because it kills the battery.

It's often not snow that I'm worried about, but mist or fogging. I find that the defogger helps a bit with mist on the rear window, so I'm hoping to have that benefit on the mirrors.

  • If you have snow available to test with, you can try leaving the car stationary with the engine running. When the engine is running, it will be generating power, so you don't drain your battery. – Kitsunemimi Feb 5 at 14:20
  • Thanks, Kitsunemimi. I'll try throwing some snow on the mirror, as suggested by the_storyteller. – user2153235 Feb 6 at 3:19
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Mirror and window heaters will not feel warm to your touch. The heater only heats the glass to slightly above freezing, which is sufficient to melt snow and ice.

An empirical test whether or not the mirror and window heaters are working is to start the car, enable the rear window heater, and leave the car idling for a few minutes.

Ideally, if you can let frost form on the mirrors and windows overnight, you will clearly be able to see the effect of the heater elements, as the mirrors and rear window will defrost within a couple minutes, while the front window should remain frosted (don't enable the front window header, or it'll defrost too.)

If frost is not available, but you have powdery snow, throw a couple handfuls of snow at the frozen window/mirror until it sticks, then start the car and enable the rear window heater. The snow should bead up into water in a couple minutes, and slide off.

  • I guess a few minutes won't kill the batteries if the engine is running, even if it is stationary and not running high RPMs. Thanks. – user2153235 Feb 6 at 3:18
  • Most car alternators put out ~50% max amperage at a cold idle speed, which is more than sufficient to power the heater systems on your car without discharging the battery. – the_storyteller Feb 6 at 17:05
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    You were right. I marked your answer as the answer. Also added my observations as a separate answer. Thanks! – user2153235 Feb 8 at 5:05
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In response to the suggestions given here, I was going to throw some snow on the mirror face and see if it melted. Tonight, however, the weather cooperated and coated the mirror with freezing rain. I drove for 20 minutes, the first 10 on the highway. The ice melted away. On the driver side, however, it's not ideal. There's a margin around the lower and right border where the ice seems to persist: enter image description here

This doesn't happen on the passenger side mirror. I recall noticing that "bum" region being particularly coated with frost in the past, when I was still wondering whether the heater worked at all. I suspect, however, that this imperfect heating isn't due to the flooding. Perhaps the solution is just a matter of replacing the mirror.

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