Bests & Worsts Reviews from Amazon

I purchased my first Tomtep M2000 (pictured on the right) and after noticing the HCHO value would never change from a 0.000 reading, I reached out to Amazon for a return. When the replacement showed up (pictured on the left), the HCHO worked but there was clearly huge discrepancies between the two meters. No matter how many times I tried recalibrating them, they each would show grossly different readings? Which one do you trust? Either one actually accurate? Obviously the CO2 readings were a little of a concern, so I asked a friend of mine whom is a licensed plumber, if he could perform a CO2 test in the same location. He came by, used his own certified equipment, and the results were 480ppm for CO2! No where near the two values the Tomtep units provided. I don’t know if I just got two bad units or what the deal is, but as it stands, these two units are not accurate, not to mention the one that doesn’t measure HCHO straight up defective. Oh and for those wondering, the time / date difference was due to the fact that I had set one, and not the other. Take what readings these unit give with a grain of salt.