A method detects coking in the inlet section of an internal combustion engine with direct fuel injection, a throttle valve and a variable inlet valve lift controller. A correction value calculated by the inlet valve lift controller as an offset value with a preset valve lift is determined. In parallel, a first quantity deviation test is carried out by which a first air ratio value is determined, which is formed from a first lambda value measured during the first quantity deviation test and a desired lambda value of the fuel combustion in the combustion chambers of the internal combustion engine, wherein, in the first quantity deviation test a load control process is carried out by way of the variable inlet valve lift controller. In a second quantity deviation test, a second air ratio value is determined which is formed from a lambda value measured during the second quantity deviation test and a desired lambda value of the fuel combustion. In the second quantity deviation test a load control process is carried out by way of the throttle valve. A comparison value is formed from the first air ratio value and the second air ratio value. Whether coking is present in the inlet section of the internal combustion engine is determined by combined evaluation of the comparison result and of the correction value.