CAP PPM-3 Problem. Always reading high. 4000 - 7000+

So I have a PPM-3 that worked great for a couple months. Lately, the reading s have been like 4000+ and of course, I havent been using any CO2, and my plants havent looked as happy. Today I walked in and it said 7000. I Unplugged it and plugged it back in and it climbed to 2000ish. Still seems high. Its in a small seeled basement with a NG hot water heater, but I used to get readings closer to 1K. FYI, I was woried my hot water heater was filling my basement with CO and CO2, so I bought a digital CO detector and it reads 0.0. I plan on getting a different CO2 monitor to just have to double check, but anyone have this issue with their PPM-3? And no, I havent went out and done the outside recalibration yet as my wiring is all up in the ceiling and I have to take it all down.