The present disclosure relates to improvements in turbocharger efficiency and more particularly to a method and system for estimating the efficiency loss of a turbocharger. The method comprises the steps of monitoring a plurality of operating parameters and determining a compressor exit temperature according to a first calibration map based on these operating parameters. An estimate of instantaneous turbocharger efficiency loss according to a second calibration map is then determined, based on the compressor exit temperature. The instantaneous turbocharger efficiency loss is used to determine an estimate of cumulative turbocharger efficiency loss during engine service. The estimate of cumulative turbocharger efficiency loss is compared with a first predetermined efficiency loss threshold and a first signal is generated if the first predetermined efficiency loss threshold is exceeded.