An on-board, computer-controlled recorder has been developed to help determine the causes of vehicle malfunctions, especially those which are ordinarily difficult to diagnose because they are infrequent, fast and/or intermittent. The computer is programmed to log the output of eight, engine-monitoring sensors every 0.1 second and temporarily store each record for 15 seconds. When a malfunction occurs, the data in the computer, as well as that collected during the malfunction, are recorded on magnetic tape. An earlier report showed the effectiveness of this recorder in diagnosing catalyst melting problems.This report describes a second application of the recorder, namely diagnosing an engine preignition problem on prototype vehicles. Preignition suddenly and unexplainably caused engine damage on these vehicles after a history of trouble-free operation. At the beginning of this work, the cause of the preignition was unknown, but a malfunction causing very advanced spark timings was suspected. One vehicle that had previously failed with preignition twice in about 3,000 miles was equipped with a diagnostic recorder and driven for 10,000 additional miles. Preignition did not reoccur so the cause of the failures on this vehicle could not be detected. A second vehicle was driven with three, different, deliberately induced malfunctions that were suspected to cause preignition. Only one of these, a faulty manifold pressure sensor, actually caused preignition; it did so rapidly under normally encountered driving conditions. Analysis of the diagnostic recorder data showed how the faulty pressure sensor induced preignition.