Mazda Repair Questions? Ask Mechanics for Answers ASAP
Hello and thank you for your question,The most common cause for the code P0431 to set is usually a left side Catalytic converter ( efficiency below threshold) . Before you consider replacing it you should monitor the front and rear oxygen sensor voltages and compare them while at idle and at a steady cruising speed. The catalytic converter is likely bad faulty if the downstream sensor is switching voltage from rich to lean (0.9 volts - 0.1 volt) in a manner like the upstream sensor.The misfire codes is as you suspect and in most cases the problem is either a failing spark plug and or coil with the coil being pretty common. The best way to check the coil is to swap it with one of the other coils and see if the misfire moves. If it dose then replace the coil. If it dose not then replace the spark plug.
The cat is not likely clogged yet but has lost its efficiency and if left alone it will likely restrict at some point. If you compare you HO2S reading and the rear matches the front then its a very safe bet the cat is bad. Its not likely coming apart yet. In most cases if it has come apart you will notice a major loss in power.
It could also be the fact that the coil is failing that the converter has been damaged. I would get the engine misfire cleared up and then clear the code and see if the Cat code returns. If it dose after is no misfire then its a pretty safe bet the Cat is failing.
Im think when its all said and done you are going to end up replacing a coil and or spark plug as well as the converter. Its a very common failure on this car. Please take a moment to rate my service so I can get credit for my time, the question remains open for follow up even after doing so, all you need to do is return to this question if need be.Thank you again.