Dual oxygen sensors before and after the exhaust catalytic converter of a motor vehicle combined with a specialized algorithm to substantially continuously compare the real time electric output of the oxygen sensors and, in response to the comparison, output an indication of the activity of the catalyst...http://www.google.de/patents/US5228335?utm_source=gb-gplus-sharePatent US5228335 - Method and apparatus for detection of catalyst failure on-board a motor vehicle using a dual oxygen sensor and an algorithm
