In order to reduce locally the gap between potential and real yields, its causes must be diagnosed. Through a case study on maize in small-scale farms of central Brazil, this poster presents an approach for this purpose, taking advantage of the advances in crop modelling. Based on an on-farm survey and on the building and use of an ad hoc crop model, this method facilitates the detection of constraints and the assessment of their impact on local yields, although it does not suppress all subjectivity in the diagnosis. STICS, an existing crop model, had to be improved for the specific purpose of the study, in order to take into account Aluminum toxicity, water excess and competition between crop and weeds for light, water and nitrogen. The local diagnosis showed that most of the yield gap was due to inappropriate crop establishment, itself resulting from shortcomings of associations of fanners in managing the collective farm machinery.