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EDUCATE YOURSELF: HERE'S THE EXPLANATION OF YOUR DIAGNOSTIC CODE

Catalyst System Efficiency Below Threshold' - P0420 Code
Catalyst System Efficiency Below Threshold (Bank 1) - is one of several trouble codes that fall into the general exhaust system code block.

All P042X codes (where "X" can be any number) refer to the catalytic converter and exhaust system. P0420 refers to the catalytic converter or - more specifically - the oxygen (O2) sensors before and after it on the bank 1 side of the exhaust system.

Exhaust System Schematic

On most vehicles, there is an O2 sensor both in front (upstream) and behind (downstream) the catalytic converter. Normally, these two O2 sensors will have different readings when the car is running. The upstream sensor should have a fluctuating oxygen level, dependent on the amount of exhaust being created and the engine's fuel consumption rate.

As the exhaust gases enter the catalytic converter, they should be "scrubbed" (or converted). When these gases are read by the second (downstream) O2 sensor, oxygen readings should be relatively steady. Thus, when your vehicle is functioning properly, the reading from the first O2 sensor to the second should be quite different.

However, if the readings taken by the O2 sensors before and after the catalytic converter are the same (or very similar), the P0420 code will trip.

Most of the time, a similar reading means that the catalytic converter is not working as efficiently as it should be. However, this trouble code can also trigger when things upstream are not doing well. As a general rule, if your engine trips a P0420 code along with other upstream trouble codes, the problem you're having likely isn't related to the catalytic converter.

However, if the P0420 code is the only trouble code - or if the other codes are all P042X codes - the likelihood of a failed catalytic converter is much higher.

Likely Causes For P0420 Trouble Code
Leaded fuel in an unleaded system will nearly always trigger this, given the differing properties of leaded fuel combustion and exhaust.
Failed O2 sensors or connections are also possible culprits.
A failed catalytic converter.
Other less likely problems: an issue with the coolant temperature sensor, an exhaust manifold leak, a spark timing issue, leaking fuel injectors, misfires, or fuel oil contamination.
Because there are a lot of possible problems that could cause a PO420 code, it's a good idea to check the (far cheaper) oxygen sensors and other items before you invest in a new catalytic converter.