This paper addresses the run-time diagnosis of delay faults in functional units of microprocessors. Despite the popularity of the stuck-at fault model, it is no longer the only relevant fault model. The delay fault model - which assumes that the faulty circuit element gets the correct value but that this value arrives too late - encompasses many of the actual in-field wearout faults in modern microprocessors. In-field wearout faults, such as time-dependent dielectric breakdown and electromigration, cause signal propagation delays which may be missed during production test time. These defects progress exponentially over time, potentially causing a catastrophic failure. Our goal is to diagnose hard delay faults (i.e., identify them as hard faults, not transients) during run-time before they lead to catastrophic chip failures. Results show that we can diagnose all injected delay faults and that prior diagnosis mechanisms, which target only stuck-at faults, miss the majority of them. © 2007 IEEE.