We recommend checking your Kia Spectra's rear oxygen sensor first; using a professional multimeter preferably one that will show wave patterns. When doing this test while your Kia is warm and running in closed loop mode, the oxygen sensor waveform reading before the catalytic converter should fluctuate. The O2 sensor waveform reading AFTER the catalytic converter should be fairly steady, anywhere between .15 to .45 volts.
If you end up needing to replace either the oxygen sensor or the catalytic converter we highly recommend using OEM parts, especially when it comes to replacing any of the oxygen sensors. These components are very sensitive and each is designed to be used with specific engine models.
Also, it is important to do properly diagnosis your Kia's P0420 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 Kia Spectra'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 P0420 trouble code fault.
Other faults to look into, besides 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 Kia Spectra'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).
