P0420 Code – Meaning, Causes, Symptoms, & Fixes

If you have a P0420 code on your OBDII reader, don’t worry. There are several things you can do to fix the issue and get your car back on the road.

P0420 Code is a diagnostic code that is associated with the catalytic converter. The code indicates that the catalytic converter is not working properly and may need to be replaced. There are several possible causes for the code, including problems with the exhaust system, the O2 sensor, or the catalytic converter itself.

We’ve outlined some of the most common P0420 Code causes and solutions in this article, so be sure to try them out before taking your car to a mechanic.

P0420 Code – Catalyst System Efficiency Below Threshold (Bank 1)

Catalyst System Efficiency Below Threshold (Bank 1) is the plain text for P0420, meaning there’s a problem with the catalytic converter on cylinder bank 1.

P0420 Code Definition:Catalyst System Efficiency Below Threshold (Bank 1)
P0420 Code Severity:Moderate. Extended driving could cause severe engine damage.
P0420 Code Repair Urgency:Troubleshoot & repair as soon as possible

The catalyst system efficiency is monitored by the PCM through signals from the oxygen sensors in front of and behind the catalytic converter.

These sensors create a voltage signal proportional to the amount of unburned oxygen in the exhaust. The PCM uses these signals to determine how efficiently the catalyst is working.

If the PCM sees that the efficiency is below a set limit, it will trigger the P0420 code.

P0420 Code Causes

The cause of a P0420 code is an issue directly related to the catalytic converter.

The catalytic converter helps to reduce emissions from your vehicle, if it’s not working properly, it can trigger a P0420 code.

Here are the possible causes triggering a P0420 code:

  • Faulty Catalytic Converter
  • Exhaust Tube Leaks
  • Intake Air Leaks
  • Incorrect Ignition Timing
  • Dirty Air Filter
  • Faulty Oxygen Sensor (O2)
  • Faulty or Leaking Fuel Injectors
  • Faulty Spark Plugs
  • Faulty Engine Control Module

Too often I see OEM catalytic converters being installed too far back on the exhaust system, even though they are designed to be installed in a specific location in the exhaust system.

If they are installed too far back, they may not reach the proper operating temperature and will not work as efficiently. This can lead to increased emissions and a P0420 error code.

If the catalytic converter is not faulty, other causes could include a clogged or restricted fuel injector, an exhaust or intake air leak, a faulty oxygen sensor, incorrect ignition timing, a dirty or clogged air filter, faulty spark plugs or a broken engine control module.

P0420 Code Symptoms

The P0420 code is a common problem, and the symptoms of a P0420 engine code are pretty severe and noticeable as the catalytic converter is not working as intended. Here are the most common P0420 symptoms:

The most common symptom of a P0420 code is a noticeable reduction in fuel economy. You may also notice that your vehicle is running rougher than usual, or that the engine seems to be lacking power. In some cases, you may also see an illuminated check engine light on your dash.

How to Diagnose P0420 Code

  • Check your air filter. A clogged air filter will restrict airflow to the engine, causing all sorts of problems. Replace the air filter if needed.
  • Look for intake air leaks. These can cause your engine to run lean, which can trip the P0420 code.
  • Check for leaks in the exhaust system. Even a small leak can cause enough extra air to enter the system to trip the P0420 code.
  • Check the oxygen sensors. If they’re dirty or faulty, they need to be replaced.
  • Replace the spark plugs. Faulty spark plugs could cause the engine to misfire and eventually lead to a P0420 code.
  • Check the catalytic converter. It could be clogged or damaged. A damaged catalytic converter needs to be replaced.
  • Inspect the fuel injectors. Clean or replace them if necessary.
  • Reprogram or replace your ECM. The engine control module could be at fault.
  • Inspect the timing belt or chain. Improper ignition timing could also cause the engine to misbehave, causing a P0420 error code.

If you can’t find the cause of your P0420 code, it’s best to take your car to a mechanic or dealership for further diagnostics.

How Do I Fix P0420 Code?

Diagnosing and repairing the P0420 code isn’t too easy because it could be caused by several different things.

It’s most likely a broken catalytic converter, but it could also be caused by a malfunctioning O2 sensor or an exhaust leak. Here are the most common repairs for a P0420 code:

  • Ensure all wiring connections are good. Look for damaged, bent, pushed out, or corroded connector pins in the wiring harness and connections to ensure it’s not a wiring issue.
  • Replace the oxygen sensor, spark plugs, fuel injectors, and air filter. If during inspection any of these parts seem to fail, replace them as necessary.
  • Replace the intake tube. If there’s an air intake leak, the cracked intake pipe or tube needs to be replaced.
  • Seal any exhaust leaks. Use a welder to seal the leak.
  • Make sure the timing is correct. Inspect the timing belt or chain and replace it if necessary.
  • Clean the catalytic converter. Either add an additive to the gas tank and take a drive, or dismount the catalytic converter and clean it thoroughly using automotive degreaser.
  • Replace the catalytic converter. If the CAT is damaged, it might have to be replaced.

How Much Does It Cost to Fix P0420 Code?

The P0420 Code is a common code, and it’s usually not difficult to fix. The repair cost varies by car make and model, but is typically less than $1,500.

There’s no exact cost to fix the P0420 code as it depends on which part is causing the code to trigger. In some cases, it could be as simple as replacing a faulty oxygen sensor which is $50. In other cases, it might require more extensive work, such as replacing the catalytic converter costing up to $1500.


Can a P0420 Code Go Away?

A P0420 code can go away if the issue is resolved. As soon as your front and rear O2 sensors read the correct and desired data inside of the exhaust system, the car will go back to normal.


The P0420 Code is associated with a vehicle’s catalytic converter. When the code is triggered, it means that the catalytic converter isn’t working properly, which can result in a number of problems for the vehicle. In some cases, the converter may need to be replaced in order to resolve the issue.

There are several things that can cause the P0420 code to be triggered, but the most common is a problem with the catalytic converter itself.

In some cases, the converter may be damaged or clogged, which will prevent it from working properly.

Other possible causes include a faulty oxygen sensor or a leak in the exhaust system.

If you have the P0420 code, it’s important to get it fixed as soon as possible. If the converter is damaged, it can cause your vehicle to emit higher levels of pollutants, which can be harmful to both you and the environment.

In some cases, it may also lead to decreased fuel economy and performance.

Fortunately, there are a few different ways to fix the P0420 code, so you can get your vehicle back to running like new again.