System and method for monitoring a catalytic converter using adaptable indicator threshold

A method for monitoring efficiency of a catalytic converter in a vehicle having an internal combustion engine connected to the catalytic converter and controlled by an engine controller having a memory for storing a malfunction indicator threshold value, the vehicle including an upstream exhaust gas oxygen sensor between the engine and the converter for providing voltage signals to the controller indicative of excursions from a stoichiometric combustion mixture and a downstream exhaust gas oxygen sensor interposed the converter and atmosphere for providing voltage signals to the controller indicative of conversion efficiency of the converter, the method comprising:

• counting switches of the upstream sensor wherein a switch occurs when the voltage signal crosses a switching threshold;
• counting switches of the downstream sensor wherein a switch occurs when the voltage signal varies in excess of a switching level;
• determining a current value for the malfunction indicator after a predetermined number of switches of the upstream sensor have been observed, the current value being a ratio based on the switches of the upstream and downstream sensors;
• calculating a historical value for the malfunction indicator based on the current value;
• and
• modifying the predetermined malfunction indicator threshold value based on the historical value.