A method for determining a relationship between the opening time for an injector of a cylinder in a combustion engine and the amount of fuel injected by the injector, characterised in that the method comprises the following steps: A) during a succession of power strokes in which the same opening time ( t open) is applied to the injector for each of them, the injector is caused to inject fuel into the cylinder in the form of late injections which take place so late during the respective power strokes that the fuel thus injected late undergoes no combustion in the cylinder and this unburnt fuel is caused to accompany the exhaust gases produced by the engine to a fuel oxidation device situated in an exhaust line from the engine, in order to be oxidised in the fuel oxidation device and thereby generate an increase in the temperature of the exhaust gases passing through the fuel oxidation device, B) a temperature increase value (Δ T BDOC-ADOC) which represents the temperature increase of the exhaust gases during their passage through the fuel oxidation device is determined on the basis of measured values from a temperature sensor provided to detect the temperature of the exhaust gases upstream of the fuel oxidation device, and from another temperature sensor (14; 15) provided to detect the temperature of the exhaust gases downstream of the fuel oxidation device, C) a calculation value ( ṁ fuel 1) which represents the mass flow of said unburnt injected fuel which accompanies the exhaust gases is determined on the basis of said temperature increase value (Δ T BDOC-ADOC), a fuel mass flow value ( ṁ fuel 2) representing the mass flow of fuel injected into and burnt in the engine and an air mass flow value ( ṁ air) representing the prevailing mass flow of air in the engine's air intake, and D) a relationship between the injector's opening time and the amount of fuel injected by the injector is determined on the basis of said calculation value ( ṁ fuel 1) and the value of the opening time ( t open) applied to the injector for said late injections. A) during a succession of power strokes in which the same opening time ( t open) is applied to the injector for each of them, the injector is caused to inject fuel into the cylinder in the form of late injections which take place so late during the respective power strokes that the fuel thus injected late undergoes no combustion in the cylinder and this unburnt fuel is caused to accompany the exhaust gases produced by the engine to a fuel oxidation device situated in an exhaust line from the engine, in order to be oxidised in the fuel oxidation device and thereby generate an increase in the temperature of the exhaust gases passing through the fuel oxidation device, B) a temperature increase value (Δ T BDOC-ADOC) which represents the temperature increase of the exhaust gases during their passage through the fuel oxidation device is determined on the basis of measured values from a temperature sensor provided to detect the temperature of the exhaust gases upstream of the fuel oxidation device, and from another temperature sensor (14; 15) provided to detect the temperature of the exhaust gases downstream of the fuel oxidation device, C) a calculation value ( ṁ fuel 1) which represents the mass flow of said unburnt injected fuel which accompanies the exhaust gases is determined on the basis of said temperature increase value (Δ T BDOC-ADOC), a fuel mass flow value ( ṁ fuel 2) representing the mass flow of fuel injected into and burnt in the engine and an air mass flow value ( ṁ air) representing the prevailing mass flow of air in the engine's air intake, and D) a relationship between the injector's opening time and the amount of fuel injected by the injector is determined on the basis of said calculation value ( ṁ fuel 1) and the value of the opening time ( t open) applied to the injector for said late injections.