Mazda Repair: mazda 2000 2.5, mazda 2000, grey stone

Question
the code is p0421 i took off my converter and there was grey stone like deposits
in it

Answer
i'm showing warm up cat efficiency below threshold (cylinder bank no.1)it could posibly be the sensor but without testing it, it's hard to say. i suggest purchasing a repair manualy for your make and model of car and following the instructions they previde for that model and test the sensor. most sensor testing is fairly easy and can be done with a digital multi meter. sorry i can't be of more help