The code P0420 indicates a catalytic converter problem. The catalyst can reduce exhaust emissions by filtering exhaust gases through fine meshes of platinum and gold.

What is Code P0420?

Code P0420 indicates a problem with the catalytic converter. The job of a catalytic converter is to break down harmful pollutants produced during the combustion cycle. The catalyst can reduce exhaust emissions by using fine meshes of platinum and gold to filter exhaust gases.   

The catalytic converter has two oxygen sensors. One oxygen sensor is located upstream (upstream) of the catalyst, and the other O2 sensor is located behind (downstream).   

If the upstream oxygen sensor is working correctly, it should fluctuate between the car’s warm-up to operating temperature and closed-loop operation. If the lower lambda probe is suitable and there is no problem with the catalytic converter, its reading should remain stable. When the oxygen sensor reads the same, the catalytic converter is not working as expected. If the downstream oxygen sensor voltage drops and fluctuates like the upstream oxygen sensor, which means the oxygen level is too high, the Powertrain Control Module (PCM) will store a P0420 fault code and turn on the warning light. Check the engine.  

Common Causes of Code P0420? 

A damaged muffler or leaky muffler. Broken exhaust manifold or leaky exhaust manifold. Drain hose damaged or drain hose leaking.  

Engine pass. Oil contamination in the catalytic converter. Faulty catalytic converter (most common).    

Faulty engine coolant temperature sensor. Oxygen sensor wiring is incorrectly connected. Make sure the engine light is on. Several things can cause the Check Engine light to come on.   

The mechanic will need to use a diagnostic tool to read the code and determine the exact cause of the problem. No power after the vehicle warms up. The engine will run poorly and may not provide enough power for acceleration. Vehicle speed cannot exceed 30-40 miles per hour.    

This can cause problems when driving on highways and highways with high-speed limits, but if you’re slower in primarily residential areas, you may not notice this symptom. The smell of rotten eggs came from the exhaust. This odor is caused by the wrong amount of oxygen in the catalytic converter, which can lead to high levels of sulfur in the fuel tank, creating an odor in the exhaust system.  

How does a mechanic diagnose a P0420 code? 

Use an OBD-II scanner to retrieve stored trouble codes from the PCM.   

Displays real-time data from downstream (rear) oxygen sensors. The downstream oxygen sensor voltage reading should be constant. Determine if the downstream (rear) oxygen sensor is functioning correctly.    

Diagnose any other codes that may be causing DTC P0420. Repair misfiring, ignition problems, and/or fuel system problems as needed. Inspect the rear oxygen sensor for damage and/or excessive wear. Test the vehicle by viewing freeze frame data to determine if the downstream (rear) oxygen sensor is working correctly.   

Check for PCM updates if the catalytic converter is faulty. After replacing the catalytic converter, a PCM update will be necessary.

The most common mistake is to replace the oxygen sensor before the diagnostic process is complete. If another component is causing the P0420 fault code, replacing the oxygen sensor will not solve the problem.

How serious is the P0420 code?   

Usually, the driver has no handling problems when the P0420 DTC is present. Other than the Check Engine light coming on, the symptoms of this DTC may go unnoticed. However, severe damage to other components can result if the vehicle is accidentally left without fixing the problem. Since there are no signs of drivability associated with the P0420 DTC, this is not considered serious or dangerous to the driver. However, the catalytic converter can be seriously damaged if the code is not corrected promptly.  

Because catalytic converter repairs are expensive, there must be a diagnosis and repair for DTC P0420 as soon as possible. What repairs can fix code P0420? Replace muffler or repair muffler leaks. If not resolved quickly, problems with the ignition system, fuel system, air intake, and misfiring can damage the catalytic converter. These components are the most common causes of a P0420 DTC.    

When replacing a catalytic converter, it should be necessary to replace it with an original unit or a high-quality oxygen sensor. Aftermarket oxygen sensors often fail, and when this happens, the P0420 trouble code can reappear. It’s also good to contact the manufacturer to see if your vehicle has a manufacturer’s warranty on emissions-related parts. 

How to Fix code P0420

When your vehicle’s catalytic converter fails or becomes faulty, the P0420 code is usually displayed. A few dollars spent on an exhaust system cleaner is a good place to start.

Regardless, run the cleaner through the exhaust system and use an automotive scan tool to clear the check engine light. Often, replacing the catalytic converter is the only solution to the problem. However, a catalytic converter problem can lead to a slew of other problems. If you don’t address those issues before installing a new catalytic converter, the damage will worsen.

Damage to your catalytic converter will continue to harm other components of your vehicle.

Be certain that you get your repair from a company that is known for performing thorough diagnostic checks. Repairing or replacing:

  • The muffler
  • The manifold
  • The exhaust pipe
  • Catalytic converter
  • Leaking fuel injectors
  • Misfire problems

Other codes may appear if your catalytic converter problem persists. You should deal with those issues concurrently with the P0420 code. They may provide insight into a larger, systemic issue.

Catalyst System Efficiency Below Threshold Bank 1 Repair Cost

Anything from a faulty sensor to a damaged catalytic converter can cause P0420. It is hard to provide a precise estimate without fully assessing the problem first.

Most shops will begin with an hour of “diag time” if you bring your car in for a diagnosis (the time spent in labor diagnosing your specific issue). This typically costs between $100 and $150, depending on the labor rate at the shop. If you hire the shop to handle the repairs, many, if not most, will deduct this diagnosis price from any necessary work. A shop will then be able to provide you with an accurate quotation for repairs to resolve your P0420 error.

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