My vitz has 1 cat convertor because it has 1 bank (one exaust) the one in front of the engine monitored by 2 oxygen sensors the sensor before the cat is a 4 wire (heated) whilst the one after the cat is a 2 wire non heated o2, it will take some 2 or 3 minutes to start working because it must reach the specified temp by his own, but ecu does not use it to switch to close loop (controlled fuel injection)
