The main thing you need to know about a CO2 sensor is that you also need a pressure sensor for its readings to mean anything. Expect to pay ~$50 for an actual

The main thing you need to know about a CO2 sensor is that you also need a pressure sensor for its readings to mean anything. Expect to pay ~$50 for an actual CO2 sensor. The cheap "organic gas" VOC sensors don't qualify. But you probably should add on one of those anyway.