Diagnostic Trouble Code P0420

P0420 “Catalyst System Efficiency Below Threshold (Bank 1)”


What is code P0420?

Code P0420 stands for “Catalyst System Efficiency Below Threshold (Bank 1)”. This diagnostic trouble code can happen for a number of reasons related to the catalytic converter. Therefore, it is important to have a qualified technician diagnose the specific problem that caused the code to be stored in your vehicle’s computer.

Get this checked out soon by a CarAdvise shop, where the work will be guaranteed, and one of our expert techs will review any repair orders.

What does code P0420 mean?

When a diagnostic scan of your vehicle indicates a code P0420, a technician understands that there is something going on with your catalytic converter. The purpose of the catalytic converter is to break down harmful emissions coming from your engine before they escape the exhaust system as pollutants.

A catalytic converter is a device in the exhaust system through which exhaust gasses pass after they leave the engine. It is a cylindrical housing with precious metals inside, usually platinum or similar metal (palladium or rhodium), that serve as catalysts for a chemical reaction. As a result, the vapors exiting the exhaust system are far less harmful than they would be without the catalytic converter.

Working in tandem with the catalytic converter is a pair of oxygen sensors that detect the amount of oxygen present in the exhaust. One oxygen sensor is placed upstream of the converter and the other downstream. Your vehicle’s computer calculates the difference in the readings between the two. If the downstream sensor indicates that there is too much oxygen coming out of the catalytic converter, the computer will store code P0420 and turn on the check engine light.


What causes code P0420?

A number of causes can trigger a code P0420. That is why an accurate diagnosis is essential. Causes of a code P0420 include:

Most likely

  • A faulty or contaminated catalytic converter ($500-$3K)
  • Bad oxygen sensor or wiring ($200-$500)

Other possibilities


What symptoms are associated with this code?

With a code P0420, your check engine light will likely come on. Your engine might lack power once it warms up and might have trouble accelerating beyond 30 miles per hour or so. You might also smell a rotten egg odor coming from the exhaust. 


How does a technician diagnose code P0420?

If a diagnostic scan of your vehicle indicates a code P0420, a technician will perform tests to pinpoint the problem. These tests include checking for any related trouble codes. The technician will also use the scan tool to read the live data of the downstream oxygen sensor to determine whether or not it has steady voltage. If your engine has a misfire or fuel system issue, that problem will need to be repaired. The catalytic converter, oxygen sensors, muffler, and other exhaust system components will be inspected for damage, decay, or leaks.


How serious is this code?

In some cases, a code P0420 might not lead to immediate drivability issues. However, ignoring the check engine light can lead to significant problems. This code in particular can allow further damage to the catalytic converter. It also means that your engine’s computer might not be getting accurate information for proper performance. While a code P0420 is not a cause for immediate concern, it should not be ignored.

If your check engine light is blinking, your vehicle should not be driven. Any other time a check engine light appears, you should have a qualified technician read the code within a week.

Get this checked out soon by a CarAdvise shop, where the work will be guaranteed, and one of our expert techs will review any repair orders.


Related Diagnostic Trouble Codes

P0421 “Catalyst 1 Efficiency Below Threshold (Bank 1)”

P0150 “O2 Sensor Circuit Bank”

P0172 “System Too Rich (Bank 1)”

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