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, such as those causing catalyst melting. 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.This report describes the use of the diagnostic recorder to help identify problems that caused catalyst melting on two prototype vehicles. When this work began, the cause of the catalyst melts was unknown, but the new ignition system on these vehicles was suspected. On the first vehicle, work with the recorder showed that two malfunctions of the electronic engine control unit caused, and a design fault of the ignition module aggravated, the catalyst melts. On the second vehicle, the recorder showed no problems, except that several attempts to run the engine with a heavily discharged battery led to engine stalls. While such stalls have caused high catalyst temperatures on other vehicles, this was not observed here because the catalyst was too cold during the stalls to burn the fuel reaching it.In these examples, the diagnostic recorder was installed on vehicles, which had previously failed, with the expectation that the same malfunction would occur again. In cases where the failure is very infrequent, this approach may be less efficient than equipping each prototype vehicle with a diagnostic recorder at the outset of testing.