Runaway CO2

Got several like this: Unit starts with "sensor initializing" might go on forever. If it does initialize, the readings never top 1.1% (my tester has shown up to 32%). PHCBI steered me to a recall/warranty repair, and swore the CO2 sensor wouldn't be needed. That didn't turn out to be true. But now I've replaced the CO2 sensor and still not getting anything close. The voltages now MOVE when I run the Zero Adjustment, but still get nothing close.