How do I fix a P0420 catalytic converter inefficiency issue?

Hey guys, I’ve got a question about my 2013 Rio LX 4-door with 76,500 miles on it. The check engine light was on for a couple of days showing code P0420, which indicates “catalyst system efficiency below threshold.” Oddly enough, it turned off on its own after two days.

The local Kia dealer can’t see me for a diagnostic appointment until a week from now and charges $90. They mentioned there’s an 8-year, 80,000-mile warranty on the catalytic converter.

Any thoughts or suggestions on what might be causing this or how to handle it before my appointment?


Firstly, there’s a possibility that the P0420 could be caused by a faulty O2 sensor, but more commonly, it indicates the catalytic converter isn’t functioning correctly. If your car has been burning oil or had significant misfires previously, it’s likely the catalytic converter is deteriorating.

The 80,000-mile or 8-year warranty for federal emissions is legitimate. However, if the check engine light is off and there’s no P0420 code currently stored, Kia may not diagnose or replace the catalytic converter. There’s also uncertainty about Kia’s response if the code was previously stored but the light is now off. In this situation, you’ll need the check engine light and P0420 code to reappear and persist until your appointment for Kia to address it.


Great advice, Thank you.

As I’m considering the potential faulty parts, it seems these components could be involved:

  • Kia Manufactured with Converter (Part Number: 28510-2BEF1). This seems to include the warm-up catalytic converter.
  • Kia Oxygen Sensor (Part Number: 39210-2B210). Front oxygen sensor.
  • Kia Oxygen Sensor (Part Number: 39210-2B220). Rear oxygen sensor.
  • Kia Intermediate Pipe (Part Number: 28600-1W850). This appears to include the under-floor catalytic converter.
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With 76,000 miles on your car, unless you’ve been driving with the check engine light (CEL) on for an extended period, I would doubt that the catalytic converter is bad. It’s more likely that there’s an issue with the O2 sensor or there could be corrosion in the sensor plug/socket connections.

In any case, continue driving the car until the P code reappears, and avoid clearing the code yourself. Then, take it back to the dealer for repairs. They won’t proceed without being able to read the codes, and it’s important to have the code(s) present to ensure you receive the correct diagnosis and repair. This also protects you in case the issue arises again after the warranty expires, as federal regulations require manufacturers to honor emissions warranties.