O3 sensor from the late 90s Alpine Air/Ecoquest/Living Air purifier days. When these worked, they worked great, shutting off the unit's ozone production when it sensed it was over a certain level but they had a high failure rate and gave up on them after a year or two.

XL15+ models would still work as a regular XL15 unit that didn't come with a sensor but the service light would stay on if i remember correctly.

I am not able to test if this sensor is working or if not if it can be repaired so it is being sold as is for parts.

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