P0420 (Catalyst System Efficiency Below Threshold Bank 1)

A trouble code of P0420 is Catalytic Converter System Efficiency Below Threshold (Bank 1). The following article will address the major causes, symptoms and possible fixes for this code. For the full rundown of what the P0420 code entails, read on.

What P0420 means for my vehicle?

The P0420 code is an indication of the inefficient performance of the catalytic converter in your vehicles emission system. The cause of this P0420 code and the resulting check engine light is a serious issue. The emissions system is important to ensure our vehicles we drive everyday are running in excellent condition.

The typical job of a catalyst is to cause chemical changes in the harmful exhaust gases without changing themselves. The gases must pass through a honeycomb-type brick present in the catalytic converter that refines these into less harmful gases for the environment.

To learn more about this complicated device check out our Ultimate Guide on How a Catalytic Converter Works

The ECM typically uses one o2 sensors at the front and one 02 sensor at the back of each catalytic converter to measure the system’s efficiency.

If a vehicle is an inline engine they will have one catalytic converter, if it is a V type engine there will be two catalytic converters. One for each bank, resulting in four total oxygen sensors.  When the efficiency of the catalytic system is low, P0420 is triggered in response.

The powertrain control module (PCM) monitors the readings of these pre and post 02 sensors. The PCM compares the readings before the catalytic convertor and after the catalytic converter.

If the PCM notices that these readings are out of an acceptable range this diagnostic trouble code will set.

Due to the inadequate efficiency in the performance levels of the catalytic converter, an increase in the level of harmful substances produced by the combustion system is observed.

Strict emissions regulations ensure that vehicle manufacturers follow guidelines that are set out to ensure that when vehicles emission systems are not functioning correctly they will set an error code.

In simpler terminology, if the check engine light is illuminated and an OBD II scanner indicates a P0420 trouble code, it is a clear indication that the system has detected the underperformance of the Bank 1 catalytic convertor. 

P0420 typically means that the convertor of the vehicle has degraded, however it can also be caused by faulty sensor readings as well.  This condition can lead to complex issues involving poor engine performance and stalling.

Signs Your Vehicle May Have a P0420 Code

Suppose there is a clog in the convertor system of your vehicle. In that case, you might notice a few symptoms in the overall performance and drivability of the vehicle that can indicate a possible P0420 trouble code.

A few noticeable symptoms are as follows:

  • Poor engine performance while driving
  • Illuminated check engine light
  • Rough idling
  • Exhaust leaks
  • Stalling
  • A failed emission test
  • Harmful gases released from the tailpipe

Causes of P0420 Trouble Code

We know that the cause of this trouble code is a fault in the convertor system of the car that directly affects the combustion cycle. This results in the release of harmful substances in the air.

There are few possible causes of a diagnostic trouble code P0420 in your vehicle that is not very noticeable.

A few of the common causes of this code can be:

  • A faulty catalytic converter
  • A fault in the oxygen sensor
  • Problems in the circuit that include damaged wires and loose connections.
  • Computer software in need of an urgent update
  • An exhaust leak
  • An improper fuel mixture
  • Vacuum Leaks
  • Worn or damaged spark plugs
  • Upstream (PRE) oxygen sensor reading incorrectly
  • Downstream (POST) oxygen sensor reading incorrectly

How Can You Fix This Trouble Code?

You may not find a quick fix for the P0420 code as a normal driver, as this requires professional assistance and specialized equipment that is not readily available for everyday use.

There can be numerous reasons behind this code appearing on a scan tool. There is a need to diagnose the code accurately and perform the necessary repairs. You need to know what to fix, from a faulty catalytic convertor to a fault in a oxygen sensor.

Since all vehicles are designed differently, consult with the factory repair center to repair the diagnostic trouble code. The catalyst needs to be replaced in most cases, which is easily available.

It is best to consult a properly licensed mechanic to get a proper diagnosis to avoid wasting your time and effort needlessly. Or if you are an accomplished do-it-yourselfer you could invest in a scan tool to help you find the cause of the fault.


Drivability Issues with the P0420 Diagnostic Trouble Code

While there is no vehicle danger imposed from this code however a few drivability concerns may accompany this code. A lack of power or  poor acceleration may be noticed if it is not repaired timely. Furthermore, a serious threat to other vehicle components is also present.

Cost of Repair

After successfully diagnosing the root cause of the P0420 DTC, you would need to fix that underlying issue. The most common repairs and their related costs are listed below. Depending on the condition, you may require one or more than one repair.

  • Replacement of oxygen sensor $275-$500
  • Repair Exhaust system $100-$200
  • New catalytic converter costs $400-$2400
  • Air fuel sensor repair costs $200-$300

Similar Codes to P0420

Misfire codes such as P0300 can also be set when this diagnostic trouble codes is present. Also, P0430 Catalyst efficiency below threshold (Bank 2) is the same code however is related to Bank 2 of the vehicle.

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