Trouble code P0421 means that the downstream oxygen sensor (this is the oxygen sensor located after the catalytic converter and on Bank 1; Bank 1 is the side of the engine on which cylinder number one is located) has detected the catalytic converter (CAT) is not working as efficiently as it should be. This is not a guarantee that the CAT is defective. It may be possible that the downstream oxygen sensor (O2), itself, is defective/damaged. So checking both oxygen sensor operations (the O2 before the CAT and the O2 after the CAT) before replacing the CAT is very important.
We recommend testing your Mazda's rear oxygen sensor first; using a professional multi-meter preferably one that will show wave patterns. This test should be conducted after your Mazda 626 has sufficiently warmed up (running in closed loop). The O2 sensor waveform reading AFTER the catalytic converter should be fairly steady, anywhere between .25 to .45 volts.The oxygen sensor waveform reading BEFORE the catalytic converter should fluctuate between 0.1 and 0.9 volts.
It is very important to accurately diagnosis your Mazda's P0421 fault. We expect the Check Engine trouble system to indicate when other emission control systems/components are defective however, often while other systems are responsible for your Mazda 626's Catalyst System Efficiency to perform below Threshold, the Check Engine light system will not indicate a problem with those system/s and simply show the CAT is not able to lower emissions enough. The fact is in order for the catalytic converter to function efficiently it relies on proper air/fuel mixture. So any fault which could alter air/fuel mixture can potentially cause a P0421 trouble code fault.
Other faults to look into, along with a defective Catalytic Converter or bad rear oxygen sensor are:
1. Damaged or leaking exhaust manifold or exhaust pipe; causing air to enter the Catalytic Converter, diminishing its efficiency.
2. Your Mazda's engine coolant temperature (ECT) sensor not working properly.
3. Defective ignition timing (if manually controlled).
4. Fuel related problems such as leaking fuel injector, high fuel pressure, defective fuel pressure relief valve (if equipped).
