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