A complete turbocharger includes the compressor housing, the centre rotating assembly and the turbine housing, ready to bolt onto the engine. A turbo core, also called a CHRA or cartridge, is only the centre section: shaft, compressor wheel, turbine wheel and bearings. With a core you reuse your existing housings, which costs less, but the housings must be in good condition and correctly clocked when refitted.

The common signs are loss of boost and power, blue or black smoke from the exhaust, a whining or siren noise that rises with engine speed, oil found in the intercooler piping or intake hose, and higher oil consumption. On modern diesels an underboost or overboost fault code is also typical.

Most turbo oil leaks start somewhere else. The usual causes are a blocked or restricted oil drain line, high crankcase pressure from a clogged breather, or a restricted air filter pulling vacuum on the compressor side. If the underlying cause is not corrected, the replacement unit will leak as well.

On many variable geometry turbochargers the electronic actuator can be replaced separately. A replacement actuator has to be calibrated to that specific turbo, because the stroke and end positions are learned values, so it should be fitted and programmed by a workshop with the correct equipment. An uncalibrated actuator normally results in underboost or overboost fault codes.