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I have a 2018 Hyundai Elantra that has a moisture sensor at the top of the windshield, the sensor had to be unplugged for the new installation as it is mounted to the glass.

As soon as we drove it, the auto defroster runs continuously, and cannot be changed to a different setting, except for off(ie: heat on feet). If you try to change from fresh air to recirculation, the button flashes three times.

Do you guys know why this would happen?

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    I don't know why it would happen, but can tell you if it's now doing this right after getting the windshield replaced, call up the company which did the repair and have them fix it. Obviously they screwed something up. It's their job to fix it right. Their work should be covered by warranty, which includes the windshield itself, but also the repair of the car. If they messed something up, make them fix it. – Pᴀᴜʟsᴛᴇʀ2 Nov 21 '19 at 0:26
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The moisture sensor is either open because of a broken wire or the connector is unseated. Another possibility is the circuit is shorted because of bent connector pins.

Either condition (constant open/short) will provide erroneous information about the state of the glass to the body computer, which in turn will lead to the computer directing the climate controls inappropriately. Thus failing to defog properly.

Understand glass guys are not troubleshooters and don't have the knowledge or tools to deal with the huge variety of connector types found on modern cars. Most cars didn't need to require any unplugging of any kind to replace front glass. Now they do, adding several different possible failure points for glass installers.

Installers I've seen are usually contractors operating under time pressure and with small profit margins. Any extra labor or parts needed to fix your issue would erase any profit from the initial glass job. The contractor also would likely have to eat this expense. So expect pushback and denials.

Therefore DIY.

Inspect the connection to the sensor carefully for bent pins, wires pulled out of the plug, or breakage. Fix it. Failing DIY, it ought to be a one hour labor job at a local shop.

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