P0420 Code: Catalyst System Efficiency below Threshold (Bank 1)


When your check engine light illuminates with the P0420 code, it means that an expensive trip to your automobile.

P0420 error code indicates that the catalytic convertor of your vehicle is underperformance and it needs proper attention.

Definition of P0420 error code

According to DTC P0420 can be defined as Catalyst System Efficiency below Threshold (Bank 1).

What exactly P0420 code is?

P0420 error code indicates that there are faults in the catalytic convertor of the vehicle. The primary function of the catalytic converter is to breakdown harmful polluting elements such as carbon monoxide, hydrocarbons, and nitrous oxide during the combustion cycle. The catalytic converter uses platinum and gold meshes to filter the exhaust fumes and thus reduce the expelled emission from the exhaust pipe.

These metals act as a catalyst and participate in the chemical reaction that turns hydrocarbon into water and carbon dioxide, carbon monoxide into carbon dioxide, and nitrous oxide into nitrogen and oxygen.

The catalytic converter uses two oxygen sensors, in which one is fixed upstream while the other is positioned downstream.

When the upstream oxygen sensor works properly in the closed-loop its reading will ideally fluctuate from the initial stage. The reading of the downstream will not change and it will remain the same as in the initial stage.


When both oxygen sensors show similar readings, it indicates that the catalytic converter is underperformance. When the oxygen level is high, then the voltage of the rear oxygen sensor decreases and fluctuates like the front oxygen sensor and powertrain control module stores the P0420 error code and turns the engine check light on.

Possible causes P0420 error code

  • Open or burnt oxygen sensor wiring
  • Bent or corroded oxygen sensor pins
  • The front oxygen sensor is not working properly
  • Faulty rear oxygen sensor
  • Faults in catalytic convertor
  • Leakage in an exhaust pipe

Possible symptoms of P0420 error code

  • Check engine light illuminates or blinking
  • Poor engine performance
  • Failed fuel economy
  • Increment in emission fuel
  • Clogged catalytic converter
  • Foul smell from the exhaust pipe

How serious is the P0420 error code?

With the P0420 error code in your vehicle the no driveability condition arrives. You are unable to notice the symptoms of this error code except that the check engine light is on. If this error does not get proper treatment, this can cause various damage issues to other components of the vehicle.


The main issue with this error code is in the catalytic converter, if the catalytic converter is not diagnosed timely, it can be seriously damaged. So you need to diagnose the root cause of P0420 and get it to repair as quickly as possible.

How to diagnose the P0420 error code?

Mechanic follows these steps to diagnose the P0420 error code:-

  • Uses an OBD-II scanner to recover error codes stored by PCM
  • Diagnose the live information of the downstream sensor to determine whether it is working properly or not.
  • Diagnoses all the codes that can generate the P0420 error code.
  • Repair misfires, ignition problem, and fuel system problem if required
  • Take the vehicle to test drive to detect freeze frame data to determine whether the rear sensor is working properly or not.
  • Check the catalytic converter from the available updates of PCM. And if required replace it and updates to PCM.

How to fix the P0420 error code?

Depending on the diagnostic results, you need to do the following steps:-

  • Replace oxygen sensors
  • Replace catalytic converter
  • Replace both sensors(if required)

The primary step: ensure that the ignition switch is off and then disconnect the battery of the vehicle.

Remove the connections of the sensors: remove the connection of the sensors to protect from the accidental burn.

Remove old catalytic converter: if you notice that the catalytic converter is damaged, remove the old one. Some catalytic converters are screwed to exhaust pipe using bolts or sometimes they are welded. You need to cut the welded pipe or unscrew the bolts.

Install new catalytic converter: remove the old catalytic converter and replace it with the new one. Use a good torque wrench to install as it can break easily if overtightened.

Most probably, an oxygen sensor fails, and the P0420 error code returns. You need to contact a professional to see your vehicle has a manufactures warranty related to emission parts.

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