RER is calculated by dividing VCO2 by VO2, therefore if the VCO2 is higher than expected but the VO2 is normal, this would explain the high RER. Alternatively, if the VCO2 is normal, but the VO2 seems low, this would also explain the high RER. In either case, the raw CO2 and O2 values should be assessed by recording known concentrations of each to make sure these are calibrated correctly. If the CO2 and O2 values are not contributing to the high RER, the flow calibration should be checked. Other factors that might cause a high RER are subject panting/breathing uncomfortable once the face mask is attached creating a higher than normal CO2 output. Another common problem can be air leaks, either from around the face mask which must be completely air tight, or within the breathing circuit.