I have changed both sample filters in my ESP Gen-3 ... they're on the back, under clear plastic screw-on covers. I'm still on my original calibration bottles after seven months. A couple of times I have had an early-morning gas cal attempt fail, but I just went to manual mode, auto-zeroed, then re-calibrated with a pass. No one has had to come out yet to fix anything on the Gen-3, AFAICR. I'm doing about 60 to 70 smogs a month on it, on average.