Abstract: The capability of model identification which bases engine performance diagnostics applied to a twin-spool and separated flow turbofan engine, was evaluated using limi ted monitoring parameters. The model identification was based on non-linear eng i ne performance simulation code, so the non-linearity of the performance was con s idered by using of the balance technology between components of engine in the in vestigation. The influence of the amount of engine operating points to the diagn osis result was discussed. The investigation shows that the method can isolate differ ent faults quantitatively with high accuracy, and the method of operating points choice is concluded.