From somewhere at the beginning of this month 4 co2 sensors began to act oddly. Values are extremely high 4000 9000 and similar. Sensors are installed in ventilation ducts.
Before it was so:
Hard to tell what might be going on from the info at hand. There’s a small chance a ground loop in the system could cause issues with the readings, you need to check that all devices share the same ground and for extra immunity tie that GND to earth at each cabinet and transformer.
We can send you some sensors to swap around for testing. Do send an email for further investigation.
Maurice