P0420 Error Code-Catalyst System Efficiency Below Threshold (Bank 1)

A P0420 code indicates a problem with the catalytic converter. The purpose of the catalytic converter  is to break down harmful pollutants created during the combustion cycle. By using fine platinum and gold meshes to filter exhaust fumes, the catalytic converter is able to reduce the emissions expelled from the exhaust pipe.

The catalytic converter has two oxygen sensors. One oxygen sensor is positioned in front (upstream) of the catalytic converter and the other O2 sensor is positioned at the rear (downstream).

If the upstream oxygen sensor is working properly, its readings should fluctuate from when the car is at operating temperature and when running in a closed loop. If the downstream oxygen sensor is working properly, and there isn’t an issue with the catalytic converter, its readings should remain steady.

When the oxygen sensors have similar readings to each other, it indicates that the catalytic converter is not working as intended. If the voltage of the downstream oxygen sensor decreases and begins to fluctuate like the upstream oxygen sensor, it means the oxygen levels are too high and the Powertrain Control Module (PCM) will store the P0420 trouble code and turn on the check engine light.

Common causes of the P0420 code?

1. Damaged muffler or leaks in the muffler.
2. Damaged exhaust manifold or leaks in the exhaust manifold.
3. Damaged exhaust pipe or exhaust pipe leaks.
4. A misfire in the engine.
5. Oil contamination in catalytic converter.
6. Faulty catalytic converter (most common).
7. Faulty engine coolant temperature sensor.
8. Faulty front oxygen sensor. 
9. Faulty rear oxygen sensor.
10. Damaged oxygen sensor wiring.
11. Oxygen sensor wiring that is not properly connected.
12. Damaged oxygen sensor connectors
13. A leaking fuel injector.
14. High fuel pressure.
15. Use of the wrong kind of fuel (using leaded fuel instead of unleaded fuel)