A method for diagnosing a condition of a system for supplying fuel to a fuel injected controlled-ignition internal combustion engine, of a type including an electric control device that makes use of an oxygen probe for closed-loop regulation of a value of air/fuel ratio admitted to combustion chambers of the engine, and according to which a signal delivered by an oxygen probe is analyzed, the method a) deducing from the signal, a change in a effective injection time making it possible to regulate richness of exhaust gases leaving the engine; b) calculating CRITERION=∫(CRITERION 1+CRITERION2+CRITERION3); c) comparing CRITERION against predetermined minimum and maximum threshold values THRESHOLD_MIN and THRESHOLD_MAX; d) diagnosing a defective condition when CRITERION is outside of a window included between THRESHOLD_MIN and THRESHOLD_MAX.