Check at site O2 reading high.. do calibration zero and span O2 run MCS100E reading O2 still high. Try disconnect FIDOR and also run MCS 100E reading O2 7-8%. Suspected s/v for zero gas FIDOR bad condition or dirty service s/v of zero gas FIDOR and install at site. Run MCS 100E result O2 reading still high. Not connected sample line FIDOR reading O2 7% note instrument air must be oil and water free suspected filter instrument air replace with spare unit ( RIZAL & EVE ABY & ANDRI )