The temporal evolution of the sawtooth crash in ASDEX Upgrade Ohmicand ECRH heated discharges is analysed using a heuristic reconnectionmodel to simulate measured ECE signals. The method turns out to bewell suited to detect islands of width above 0.1-0.2 times the mixingradius; for smaller island widths, we cannot distinguish between theresistive and the ideal mode. Thus, the nature of the precursor oscillationcannot be fully resolved. In contrast, we find that during the crashphase, reconnection is incomplete and a large island persists thatfully reconnects on a slower time-scale after the crash. The observationsare consistent with a recent sawtooth model.