When a FLT24 is displayed it usually means that there has been a calibration fail, this may be from a bad manual calibration, or because the environment in which the sensor is installed has a high ambient level of CO2. Periodically, the Ax60+ performs an automatic zero calibration, if there is too much CO2 in the air when this calibration happens, sometimes it can fail. The first task is to check that the environment around your Ax60+ for high levels CO2, this is done by ventilating the area and checking for any slow leaks.
If this does not seem to be the problem then check if the fault is latched or not, you can tell it's latched when there is an * instead of a > on the top line of the display. Press the cycle button to scroll through each sensor, if you see * displayed, press and hold the green accept/test button for 3 seconds until the * turns into a >. This will unlatch the fault and allow it to be cleared.