A timed extension of the discrete-event systems failure diagnosis approach of [1], [2], is proposed. The diagnoser is a timed automaton, constructed off-line, from a timed automaton system model. The construction procedure is illustrated through a practical example. A notion of Δ-diagnosability for timed languages, is defined. Necessary and sufficient conditions linking Δ-diagnosability, to the structure of a constructed diagnoser are given.