P0420 Engine Trouble Code Help: Diagnostic & Repair

What Does the P0420 Code Mean?

The P0420 engine code indicates the catalyst system efficiency is below the threshold (Bank 1). A P0420 code indicates a problem with the catalytic converter, more specifically. 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.

Does the P0420 Code Need to be Addressed Immediately?

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 could be 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 and your vehicle will not pass an emissions inspection. Because a catalytic converter repair is expensive, it is vital the P0420 trouble code is diagnosed and repaired as soon as possible. It is recommended to wait no longer than a few weeks to have the P0420 code inspected and repaired.

P0420 Code Common Symptoms

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:

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

Common P0420 Engine Code Causes

There are several common causes of the P0420 engine code, including the following:

• 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 the catalytic converter
• Faulty catalytic converter (most common).
• Faulty engine coolant temperature sensor
• Faulty oxygen sensor (front or rear)
• 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)

How to Repair the P0420 Code

Here are the most common repairs needed after the cause of the P0420 code has been diagnosed:

• 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 the 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)

How Does a Mobile Mechanic from YourMechanic Diagnose the P0420 Code?

When choosing YourMechanic, a mobile mechanic in your area will come to your home or office at your convenience to perform an inspection and fix the P0420 code.

A mechanic from YourMechanic will follow these general steps to diagnose the problem with the P0420 code:

• Use an OBD-II scanner to retrieve trouble codes stored by the PCM.
• View 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.
• Diagnose any other codes that could be causing the P0420 trouble code.
• Repair misfires, ignition problems, and/or fuel system problems if necessary.
• Inspect the rear oxygen sensor for damage and/or excessive wear.
• Test drive the vehicle while viewing the freeze frame data to determine whether or not the downstream (rear) oxygen sensor is working properly.
• Check 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.

Mistakes Made When Diagnosing the P0420 Code

The most common mistake when diagnosing the P0420 code 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.

Other Considerations to 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 the most 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.

If your P0420 code is currently set, get a quote and schedule a P0420 diagnostic with YourMechanic below.