P0420 OBD-II Trouble Code: Catalyst System Efficiency Below Threshold (Bank 1)

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P0420 is a diagnostic trouble code (DTC) for "Catalyst System Efficiency Below Threshold (Bank 1)". This can happen for multiple reasons and a mechanic needs to diagnose the specific cause for this code to be triggered in your situation. Our certified mobile mechanics can come to your home or office to perform the Check Engine Light diagnostic for $114.99 . Once we are able to diagnose the problem, you will be provided with an upfront quote for the recommended fix and receive $20.0 off as a credit towards the repair. All our repairs are backed by our 12-month / 12,000-mile warranty.

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P0420 code definition:

Catalyst System Efficiency Below Threshold (Bank 1)

What the P0420 code means:

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?

  • Damaged muffler or leaks in the muffler.
  • Damaged exhaust manifold or leaks in the exhaust manifold.
  • Damaged exhaust pipe or exhaust pipe leaks.
  • A misfire in the engine.
  • Oil contamination in catalytic converter.
  • Faulty catalytic converter (most common).
  • Faulty engine coolant temperature sensor.
  • Faulty front oxygen sensor.
  • Faulty rear oxygen sensor.
  • Damaged oxygen sensor wiring.
  • Oxygen sensor wiring that is not properly connected.
  • Damaged oxygen sensor connectors
  • A leaking fuel injector.
  • High fuel pressure.
  • Use of the wrong kind of fuel (using leaded fuel instead of unleaded fuel)

4 Common symptoms of a P0420 code:

Though there are often no noticeable symptoms of a failure within the catalyst system, including drivability problems, there are 4 common indicators of the P0420 code:

1. Check Engine Light is on. Many things can trigger the check engine light to turn on. A mechanic will need to use a diagnostic tool to read the code and pinpoint the exact cause of the problem.

2. Lack of power after vehicle warms up. The engine will not run as well and may not provide enough power for acceleration.

3. Speed of vehicle may not exceed 30-40 mph. This may cause issues when driving along highways and freeways with higher speed limits, though you may not notice the symptom if you drive slower through mostly-residential areas.

4. Rotten egg smell coming from the exhaust pipe. This smell occurs from the incorrect amount of oxygen in the catalytic converter, which leads to excess sulfur in the fuel tank, creating the odor from the exhaust system.

How does a mechanic diagnose the P0420 code?

  • Uses an OBD-II scanner to retrieve trouble codes stored by the PCM.
  • Views the live data of the downstream (rear) oxygen sensor. The downstream oxygen sensor voltage reading should be steady. Make a determination as to whether the downstream (rear) oxygen sensor is working properly.
  • Diagnoses any other codes that could be causing the P0420 trouble code.
  • Repairs misfires, ignition problems, and/or fuel system problems if necessary.
  • Inspects the rear oxygen sensor for damage and/or excessive wear.
  • Test drives the vehicle while viewing the freeze frame data to determine whether or not the downstream (rear) oxygen sensor is working properly.
  • Checks for available updates of the PCM if the catalytic converter is faulty. After the catalytic converter is replaced, updates to the PCM will be required.

Common mistakes when diagnosing the P0420 code:

The most common mistake is replacing the oxygen sensors before completing the diagnostic process. If another component is causing the P0420 trouble code, replacing the oxygen sensors will not fix the problem.

How serious is the P0420 code?

It is common for a driver to experience no driveability problems when the P0420 trouble code is present. Other than the Check Engine Light being on, the symptoms of this trouble code may go unnoticed. However, if the vehicle is left in error without addressing the problem, serious damage could occur to other components.

Because there are no driveability symptoms associated with the P0420 trouble code, it is not considered serious or dangerous to the driver. However, if the code is not addressed in a timely manner, the catalytic converter could be seriously damaged. Because the catalytic converter is expensive to repair, it is vital the P0420 trouble code is diagnosed and repaired as soon as possible.

What repairs can fix the P0420 code?

  • Replace muffler or repair leaks in the muffler.
  • Replace exhaust manifold or repair leaks in the exhaust manifold
  • Replace exhaust pipe or repair exhaust pipe leaks
  • Replace catalytic converter (most common)
  • Replace engine coolant temperature sensor
  • Replace front or rear oxygen sensor
  • Repair or replace damaged wiring to oxygen sensor(s)
  • Repair or replace oxygen sensor connector(s)
  • Replace or repair fuel injector(s) that are leaking
  • Diagnose any repair any misfiring problems
  • Diagnose and repair any other related trouble codes that have been stored by the Power Control Module (PCM)

Keep in mind:

Problems with the ignition system, fuel system, air intake, and misfires can damage a catalytic converter if they are not addressed quickly. These components are most the common causes of the P0420 trouble code. When replacing a catalytic converter, is it a good idea to replace it with an OEM unit or an oxygen sensor replacement that is of high quality.

It is common for aftermarket oxygen sensors to fail, and when they do, the P0420 trouble code may return again. It is also a good idea to contact the manufacturer to see if your vehicle has a manufacturer warranty on parts that are related to emissions.

Need help with a P0420 code?

YourMechanic offers certified mechanics who will come to your home or office to diagnose and repair your vehicle. Get a quote and book an appointment online or speak to a service advisor at 1-844-668-2785.

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Related questions

O2 sensor failed
Hi there. A p0420 usually means the catalytic convert is not performing as well as it needs to. Over time, the catalytic converters fill up with carbon and and will not filter out the exhaust gas as efficiently as they...
Very hard to start vehicle
Hello. This code is also related to the catalytic converter (https://www.yourmechanic.com/services/catalytic-converter-replacement) not working properly. This can also happen when the ground wires connected to the O2 sensors (https://www.yourmechanic.com/services/oxygen-sensor-replacement) are not properly hooked up causing them to send erroneous info to...
My car shows a P0420 code and the engine coolant temperature reading (via torque pro app and elm327 Bluetooth reader) stayed at 84.2 degrees
Hi there - "boundary measured engine temperature," or the lowest temp reading a sensor can transmit, can be different based on each temperature sensor. The issue here is what is the true engine temperature? Is the thermostat getting the engine...

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