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Tesla Vision fails as owners complain of Model 3 cameras fogging up in cold weather
(www.notebookcheck.net)
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It's probably internal fogging because the units aren't well sealed.
They aren't normally sealed in most integrations, but will often have heater wires around them or embedded in the windshield that will heat up the windshield/sensor enclosure, just like the defog on a rear window or mirrors.