High-temperature sintering can cause loss of activity in the catalyst of an automotive emission control system. The elevated temperature required to sinter the catalyst could arise from a combination of a warmed-up converter plus unusually high engine-out carbon monoxide or unburned hydrocarbon “fuel.” This unburned “fuel” could result from sustained engine misfiring due to faulty ignition or from a prolonged high-speed, closed-throttle coast. Such abnormal “fuel” input to the converter has been prevented in all 1975 General Motors production vehicles by using the more reliable High Energy Ignition system and by properly controlling engine operating parameters during a closed-throttle coast.During the Corporate-wide program to develop systems to limit catalytic converter temperature, a “guard” converter system was evaluated as one of many candidate solutions. The guard system uses a small monolith catalyst to oxidize a controlled fraction of the unburned “fuel” in the engine exhaust stream, then rejects heat from this partially treated exhaust before passing the gas on to the main bead-bed converter located downstream.In tests with a bead bed and a guard system installed on a 1974 pre-converter vehicle, maximum bead-bed temperature during closed-throttle coasts was lowered significantly, with no penalty in emissions on the 1375 Federal Test Procedure. However, the reliability of the guard system is limited by deterioration of the very small monoliths. This deterioration must be improved substantially before the guard system is a viable production candidate.