a catalytic monitoring method for an engine having two engine banks of which each coupled to one of two catalytic converters using first and second exhaust gas oxygen sensors , respectively upstream and downstream of one catalytic converter . third and fourth exhaust gas oxygen sensors are respectively coupled upstream and downstream of the other catalytic converter . switch ratios are determined for each of the engine banks based on the switching ratios of each upstream and downstream pair of exhaust gas oxygen sensors . a combination of the switch ratios is used to determine overall catalytic converter system performance .