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The Cluster/CODIF sensor is a time-of-flight instrument that measures the ion composition over the energy range 40 eV e⁻¹ to 40 keV e⁻¹. It operated for 4 yr on S/C 1, 9 yr on S/C 3, and is still operational on S/C 4, after more than 12 yr. During this time the total ion detection efficiency has decreased by a factor of 50. In this paper, we describe the methods used to track the efficiency changes throughout the mission for the three different spacecraft and for the different ion species. The methods include calculations of the efficiencies using rate data collected in the instrument, comparisons with other instruments on the Cluster satellites, and checks based on geophysically reasonable assumptions.