What does the error message 'FAC' mean on the MP series meters?
Meaning of FAC error message on MP meters
The MP Series meters have built in calibration limits. A nominal "FAC" value is an ideal value stored by the factory. Attempts to calibrate too far (± 10% or ± 1 pH unit) from this value causes the displayed value to be replaced with "FAC". If the CAL/MCLR key is pushed, the value is accepted, and the original default factory calibration for this measurement is shown. The need to calibrate so far out that "FAC" appears indicates a procedural problem, incorrect standard solution, a very dirty cell cup, or an aging pH/ORP sensor.