A CO2 refrigeration system can include a condenser, multiple fans, and a controller. The condenser can be configured to cool CO2 and the multiple fans can be configured to affect cooling operations of the condenser. The controller may be configured to obtain a temperature value of CO2 output by the condenser. The controller may be configured to determine if the condenser is operating properly using the temperature value of the CO2. The controller may be configured to obtain values of input current and input voltage provided to the multiple fans. The controller can determine a number of in-operational or faulty fans of the multiple fans using, at least in part, the values of the input current and the input voltage and a model that relates input current to input voltage for known numbers of in-operational or faulty fans.