Yes with the SGP30 we made similar experience. The SHT3x were better.
And the Senseair S8? Sounds like AliEx is pretty hit or miss, mostly miss, so if you have any other recommendations I am all ears 
Not sure what I can recommend. We also sell parts in our shop but they are a lot more expensive than AliExpress because we cover our overheads and research and development with them.
You can check other sellers with high reputation like digikey, Adafruit, Mouser etc.
How about bme680 or 688 ?
We don’t have experience with these sensors.
You should try - I love them… BME680 is a Temperature, Humidity, VOC, and Pressure sensor.
I’ve been using the BME680 with ESPHome in an AirGradient Pro enclosure using the BSEC version.
I’ve had mixed results.
Breath VOC seems basically useless along with the CO2 equivalent, as they vary widely from the S8 values, although sometimes they line up (Yellow: BME, Blue: S8)
Temperature is consistently 3 F lower on BME
IAQ is much different from the calculated values I have based on PM2.5 readings
I do really like having barometric pressure though.
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Has anyone found sellers on Aliexpress that provide good quality sensors?
Looks like otherwise the sensors are either unavailable from major electronics component distributors (in the US), or they are more expensive.
If there is a decent Aliexpress supplier that could cut the cost of sensors down from $115 to $40-50…
Which exact sensors do you like to purchase?
Would be looking to buy a full set of the sensors for an AirGradient PRO.
I got the PRO PCB update for my 4.2, but figured instead of desoldering all the sensors and soldering them on the new board I would just add new sensors to the board and make a second AirGradient.
I’m going to order from the AG shop, and one suggestion is a full “kit” of sensors + cables. I almost didn’t see that cables weren’t included for the sensors, and almost missed that I should order the right angle USB cable as well. Basically everything needed if you have the main board, and printed your own case. I guess maybe screws would be good to include also!
Which kit do you plan to order? The Open Air and One kits include the USB cable and screws.
I am trying to build a One, I have the PCB already from when it was first released, to upgrade my DIY 4.2, but now just want to add sensors rather than unsolder and resolder. I also already have a case printed. So I only need the sensors / cables. Screws aren’t critical but nice to have I guess.
Just send me a message through our support form after you order and we can add the screws etc to it.