P0420 Code – Meaning, Causes & How to fix it

P0420 is a well-recognized error code within the automotive industry, notorious for its persistence. Luckily, this article is designed to guide you through understanding the implications of this error code, exploring its possible causes, and navigating potential fixes.

Drawing on our experience, we acknowledge that resolving P0420 can be challenging without appropriate knowledge. This complexity arises as the error can take a significant amount of time to reappear, even after parts replacement and code resetting. Throughout this blog post, we will address these issues and more, providing you with comprehensive insight into the P0420 code.

What Does The P0420 Code Mean?

The P0420 code indicates the Engine Control Module (ECM) has detected reduced catalytic converter efficiency on Bank 1. This usually points to a bad catalytic converter, but it can also be triggered by a faulty oxygen (O2) sensor giving false readings.

The ECM relies on two O2 sensors, one before and one after the catalytic converter, to monitor how effectively the converter is working. When the readings from these sensors are too similar, it indicates that the catalytic converter isn’t functioning correctly, which triggers the P0420 code.

In most cases, this issue is caused by a bad catalytic converter. However, an incorrect air-fuel mixture can also reduce the converter’s efficiency, sometimes temporarily, sometimes permanently, leading to the same error code.

Code Definition

P0420 – Catalyst System Efficiency Below Threshold (Bank 1)

What Causes A P0420 Code?

The most common cause of the P0420 code is a faulty catalytic converter. However, it can also be triggered by issues such as a bad oxygen sensor, an exhaust or intake leak, or an incorrectly placed catalytic converter.

While a defective catalytic converter is often the culprit, I’ve seen plenty of cases where the catalytic converter was new but not an OEM part. Some cheaper aftermarket converters simply aren’t efficient enough, which can cause the code to return and may eventually require replacing it with a genuine OEM unit.

I’ve also seen situations where non-OEM catalytic converters were installed too far down the exhaust system, near the last muffler, instead of close to the engine. When that happens, the converter doesn’t heat up properly, leading to the P0420 code.

Additionally, an incorrect air-fuel mixture, caused by a faulty injector, high fuel pressure, a damaged sensor, or other fuel system problems, can harm the catalytic converter over time.

Main causes of the P0420 code:

1. Damaged catalytic converter (most common)
2. Oil contamination in the catalytic converter
3. Incorrect placement of the catalytic converter
4. Damaged oxygen sensor or faulty wiring (upstream or downstream)
5. Exhaust or intake leak
6. Using the wrong type of fuel (leaded instead of unleaded)
7. Rich or lean air-fuel mixture (damages the converter)
8. Engine misfires (damages the converter)
9. Faulty engine control module (rare)

How To Fix The P0420 Code

If your check engine light is on and you’re seeing code P0420, it means your car’s catalytic converter system isn’t working as efficiently as it should. This is a common issue on many vehicles and can be caused by a variety of problems.

Before replacing any parts, it’s essential to diagnose the P0420 code properly. Use an OBD2 scanner to confirm the trouble codes, check oxygen sensor readings, and inspect for any exhaust leaks.

Here are the most effective ways to fix a P0420 catalytic converter efficiency code:

1. Clean the Catalytic Converter
   Try using a catalytic converter cleaner or fuel additive to remove carbon buildup that can affect the catalytic converter’s performance.
2. Do a Proper Diagnosis
   Before replacing any parts, perform a thorough diagnosis to identify the exact cause of the P0420 code. You will find everything you need to know in this article
3. Replace the Catalytic Converter
   If the diagnosis confirms the converter is worn out or clogged, replacement may be necessary.
4. Replace the Front or Rear Oxygen Sensor (O2 Sensor)
   Faulty O2 sensors often cause incorrect readings, triggering the P0420 code.
5. Repair Damaged Sensor Wiring
   Check and repair any broken, corroded, or shorted wires connected to the oxygen sensors.
6. Replace Damaged Engine Sensors
   Faulty sensors such as the MAF (Mass Air Flow) or MAP (Manifold Absolute Pressure) sensor can cause incorrect fuel-air mixtures that lead to P0420.
7. Fix High Oil Consumption
   Burning oil can contaminate the catalytic converter and reduce its efficiency.
8. Repair Engine Misfires
   Misfires send unburned fuel into the exhaust, which can damage the catalytic converter.
9. Correct Lean or Rich Fuel Mixtures
   An incorrect air-fuel ratio can cause incomplete combustion, leading to the P0420 error.
10. Replace the Engine Control Unit (ECU) (Rare)
    In rare cases, a faulty ECU can misread sensor data, triggering a false P0420 code.

Common Symptoms of A P0420 Trouble Code

The most common symptom of a P0420 code is the check engine light appearing on your dashboard. In some rare cases, you might also notice engine misfires, increased fuel consumption, or even a sulfur-like odor coming from the exhaust.

However, in most situations, the check engine light is the only noticeable sign. The car will usually run normally, with the P0420 code stored in the Engine Control Module (ECM) or Powertrain Control Module (PCM).

How serious is the P0420 code?

The P0420 code isn’t a severe issue, and in most cases, it won’t cause any major problems with your engine.

The main risk is if the catalytic converter becomes severely damaged and its internal parts break loose, potentially blocking the exhaust flow, but that’s pretty uncommon.

That said, a P0420 code does mean your car is emitting higher levels of pollutants, which isn’t ideal for the environment. So while it’s not an urgent repair, it’s still a good idea to address it as soon as possible.

What are the common P0420 diagnosis mistakes?

The most common mistake when dealing with a P0420 code is replacing the oxygen sensors without properly diagnosing the problem. In reality, this code is most often caused by a failing catalytic converter, sometimes damaged by other engine issues like misfires or an air/fuel mixture that’s too rich or too lean.

Start by scanning for any other trouble codes. If you see codes like P0300 to P0308, that means your engine is misfiring, and those misfires could be triggering the P0420 code. In that case, fix the misfire issue first, clear the codes, and see if P0420 returns.

If you find codes such as P0171, P0172, P0174, or P0175, your engine is running either rich or lean. This can be caused by problems like a faulty fuel injector, incorrect fuel pressure, a bad MAF sensor, or even a malfunctioning coolant temperature sensor. Address the air-fuel mixture issue first, then monitor if the P0420 code reappears.

If there are no issues with the engine and no other trouble codes are present, you can try using a catalytic converter cleaner before replacing the converter itself. There are plenty of options on the market, so it’s worth choosing a top-rated product to give it a try.

Common P0420 Code Causes by Car Model

The P0420 trouble code is more common in some car models than others. Certain car models are known to experience this diagnostic trouble code. Here is a list of the most common causes by car brands.

Remember that these are only general guidelines, and you should make a diagnosis before replacing any parts.

1. Chevrolet

If you experience a P0420 code in your Chevy, it is often caused by an exhaust leak upstream of the catalyst. It can also be caused by a faulty downstream oxygen sensor (rear) or a faulty catalytic converter.

Chevy engines are known to leak around the exhaust manifold, so you should definitely take a look there if you can find some strange blowing sounds when your engine is running. You will also find P0420 recalls for GM vehicles.

2. Toyota & Nissan

The most common cause for P0420 in Toyota and Nissan cars is a bad catalytic converter. You should also ensure your engine is not burning motor oil.

Check for vacuum leaks and exhaust leaks first. Then check for any blue smoke coming from the exhaust pipe. Blue smoke coming from the exhaust pipe is a sign that you might have an issue with the crankcase ventilation, or even something as bad as a damaged turbocharger or bad piston rings.

If you do not notice any blue smoke at any RPM, it is most likely that your catalytic converter is worn out. You may find similar issues on Nissan cars.

2. Ford

Ford engines often have vacuum leaks or a broken solenoid for the EGR or other components, which causes a faulty air-fuel mixture and then causes the trouble code.

Check the code memory with a diagnostic scanner to see if you can find any trouble codes about the air-fuel mixture. If everything looks fine, check for exhaust leaks.

Diagnose and replace the catalytic converter if you can’t find any trouble codes or other problems with the air/fuel mixture.

3. Subaru & Honda

If you find a P0420 code in your Subaru or Honda, it’s most likely the catalytic converter to blame. It can also be caused by an exhaust leak or a bad oxygen sensor.

Check for vacuum leaks or other fuel mixture-related engine codes. Check for any exhaust leaks before the catalytic converter. However, the most common problem with Subaru engines is the catalytic converter. They seem to wear down fast on these boxer engines.

Acura also has some P0420 recalls you should look into, because they share many similarities with Honda.

4. Volkswagen (VW) / Skoda / Seat / Audi A4 1.8T / V6 2.4

There is a Check Valve/PCV Valve under the intake manifold on many VAG engines, which is the first thing you need to check if you see this trouble code. You must also ensure that the crankcase ventilation is free from sludge, because clogged-up crankcase ventilation causes the engine to burn oil, which in turn clogs the catalytic converter.

Check for exhaust leaks around any flex pipes on the exhaust pipe – this is a common cause – and check for any engine codes related to the oxygen sensors. If no problems were found, replace the catalytic converter. It’s a widespread problem on both the 1.8T and the V6 petrol engines.

P0420 Diagnosis

P0420 is usually caused by a faulty catalytic converter, as mentioned before. You should always diagnose it properly with the methods below before replacing any parts.

1. Connect an OBD2 Scanner and look for related trouble codes. Repair any trouble codes related to the ignition or fuel first.
2. Check the live data to see the front and read the O2 sensor voltage signals. The car engine should be scorching, and the front sensor should fluctuate between 0–1 volts, and the rear should be steady at 0.7–0.9 volts. If you get similar readings, the catalytic converter may be defective.
3. Heat up the engine, then check the temperature in front and rear of the catalytic converter with a laser thermometer. If the engine is hot and there is no difference in temperature in front of and behind the catalytic converter, your catalytic converter may not be working as intended.
4. If the catalyst is installed in a way that’s accessible, it may be worth removing the pipe from one end and inspecting the catalyst for visible damage.
5. If everything points to a faulty catalytic converter, replace it. If you can’t find any problems with the temperature or voltage, or with a visual inspection, you should try to repair other related trouble codes, then clear the codes and try again.
6. If you still can’t find any problems, make sure it is a genuine OEM catalytic converter and is installed in the original place. If everything seems fine, replace the catalytic converter.

What could damage the catalytic converter?

The catalytic converter reduces the three main causes of pollution produced through gasoline combustion – nitrogen oxide (NOX), carbon monoxide, and hydrocarbons. It does this by converting these harmful substances into less noxious ones that are easy on our health and contribute to clean air quality.

There are a few things that are known for damaging the catalytic converter, which can cause the P0420 trouble code; here are the most common ones:

• Misfires
• Oil consumption
• Exhaust leak
• Intake leak
• Rich mixture
• Lean mixture
• Bad ECM/PCM

P0420 Repair Cost

There is no fixed cost to fix the P0420 code because of the wide range of different causes. However, it is often caused by a faulty catalytic converter, which usually costs between $500 to $1500.

The estimated cost of repairing the code is the following. The prices include parts and labor work at a workshop. The costs do not include diagnosis costs.

• Catalytic Converter Replacement – $500 to 1500
• Front Oxygen Sensor Replacement – $150 to $300
• Rear Oxygen Sensor Replacement – $150 to $300

Can I remove any parts to get rid of the P0420 code?

You cannot just remove parts to fix this trouble code, because it will likely give you another trouble code or other issues. In theory, you could reprogram the engine control unit to remove the catalytic converter’s monitoring system. However, doing so would be illegal in most states and countries, so even if it’s theoretically possible, you shouldn’t do it.

If you want to reprogram the function, you can also remove the catalytic converter completely. Remember that if you remove the catalytic converter, you will most likely not pass any emission tests and you may experience legal issues.

There is also another way to trick the engine control module by installing the rear oxygen sensor inside of a small pipe. This method can work if you desperately want to get rid of the P0420 code, but it’s not a recommended method, and it may get you into trouble.

FAQs

I hope you now have the knowledge to understand what the PO420 code is and how to remove it. If you liked this article, don’t forget to check out our other articles and our YouTube channel!

Learn more:

• P0430 – Catalyst System Efficiency Below Threshold (Bank 2)
• How to Bypass Oxygen Sensors – Can & Should You Do it?
• Do Diesel Engines Have Catalytic Converters?