The Engine Fault Code P0420 : Symptoms – Causes and Solutions

The Engine Fault Code P0420 : Symptoms – Causes and Solutions.  P0420 is without a doubt one of the most commonly encountered car engine diagnostic trouble codes (DTC). DTC is also known as engine codes, fault codes, troubleshooting codes, and so on. Whatever words are used, one thing is certain: something is wrong. But don’t worry. This article will go over the meaning, symptoms, causes, and solutions for fault code P0420.

*P0420 is a generic code used by many car manufacturers, including Honda, Toyota, Nissan, Chevrolet, Ford, Dodge, and BMW. Please leave a comment if you found this guide useful. If readers find this information useful, we may consider writing P0420 articles tailored to specific engines or brands.

The Engine Fault Code P0420  Symptoms - Causes and Solutions

DTC P0420 Definition

P0420 is a diagnostic trouble code that means “Catalyst system efficiency below threshold (bank 1).”

This refers to the engine’s catalytic converter(s), which are in charge of breaking down pollutants in the exhaust. Catalysts, also known as cats, are an important part of an engine’s emissions system because they help to reduce harmful gases. DTC P0420 indicates that the catalytic converter on bank 1 is not fully operational.

However, it is not always the catalytic converter that fails. There are numerous other causes that could result in fault P0420. However, the code indicates that something is wrong with the catalyst/emissions system.

Symptoms of Fault Code P0420

Among the symptoms of P0420 catalyst system efficiency below the threshold are:

  • Engine light on (CEL, MIL, etc)
  • The odor of raw exhaust
  • Inadequate fuel economy
  • Misfires

AFR lean or fat?

The most obvious indicator will be scanning the engine computer via the OBDII port and seeing the code appear. It’s common for DTC P0420 to throw a check engine light. A raw exhaust smell can be caused by exhaust leaks or poorly functioning catalytic converters. This smells like rotten eggs or sulfur. These are the most common P0420 symptoms. You may notice poor fuel economy in some cases.

Misfires, air-fuel ratios too lean or rich, power loss, and misc. symptoms may also occur. However, these are usually symptoms of another underlying problem that could be causing DTC P0420. Anyway, we’ll return to this point in the section on the causes of fault code P0420.

DTC P0420: How Serious Is It?

Engine fault P0420 is not usually an emergency. The problem is unlikely to cause serious drivability or reliability issues. However, your engine may be emitting more harmful emissions because the catalyst system on bank 1 is underperforming. As a result, along with the check engine light, you are unlikely to pass emissions inspections.

The safest option, as with any engine problem, is to diagnose and repair the P0420 problem as soon as possible. So we’re not suggesting that you ignore these issues for weeks or months. Rather, driving with engine code P0420 is likely safe. Use your best judgment. Play it safe if the car feels dangerous or if there are other engine codes, serious symptoms, and so on.

For example, if you notice the engine running lean, it’s best not to drive it or, at the very least, keep the throttle input and RPMs low. Lean engine operation may expose the engine to more serious problems. Driving with low power and other drivability issues can also be dangerous.

Causes of Engine P0420

The following are the most common causes and reasons for engine DTC P0420:

  • Catalytic converters that are worn or failing
  • Cat scarcity
  • Leaks in the exhaust system

The P0420 code indicates that the bank 1 catalyst system efficiency is below the threshold. Something is physically causing the system to fail, or another issue is leading the engine to believe there is a problem with the catalytic converter (s).

The most common cause of DTC P0420 is worn, failing, or missing catalytic converters. It’s not uncommon for them to lose effectiveness as they age, particularly after 10 years and 120,000 miles. Catalytic converter theft is unfortunately common, so you may have missed the cats entirely.

Another common cause of the fault code is exhaust leaks. Rust and holes can form in the exhaust system. Exhaust manifold gaskets or manifold cracks can also occur.

Otherwise, look for defective O2 sensors (or another similar sensor). The cats may be in good working order, but a faulty O2 sensor can trick the car into thinking something is wrong with them. Misfires and high AFRs can be underlying problems that lead to catalytic converter failure.

Causes that are less common

The following are some of the less common causes of engine fault code P0420:

  • O2 sensor failure
  • Misfires

AFRs with a lot of Money

Catalysts may be in excellent working order. A faulty O2 sensor can cause the engine to believe there is a problem with the cat, resulting in P0420 being displayed. Misfires and excessive air-fuel rationing aren’t always the direct cause of the code. Rather, these could be underlying issues that eventually lead to cat failure. This is due to the fact that it allows excess fuel into the exhaust, which can eat away at the catalysts and cause premature failure.

P0420 Diagnosis Code

It’s critical to get the whole picture when diagnosing any engine problem. If other diagnostic trouble codes are present, they may be related to code P0420. We’ll get back to that in a minute. Here’s a quick rundown of how to troubleshoot and diagnose the P0420 engine code:

Ascertain that P0420 is the only engine code present.

Clear the code and drive for approximately 15 minutes. Check to see if the code is returned. If not, repeat the process a few times to be sure. If DTC P0420 appears, there is an actual problem.
Examine the exhaust system physically for any damage, holes, rust, or other potential leaks. Don’t forget about the exhaust manifold, which is a common source of leaks.
Examine the voltage reading from the O2 sensor. On most engines, standard readings should be around 0.4 to 0.5 volts. If it’s wildly off or bouncing around, it’s most likely due to a problem with the catalytic converter.
Engines occasionally experience minor hiccups, and fault codes appear at random. In these cases, simply clearing and deleting the code may be sufficient to resolve the issue. Otherwise, it’s time to figure out what’s causing DTC P0420. The steps outlined above are fairly effective for exhaust leaks and cat failure. However, if you’re getting other engine codes, you’ll need to be a little more creative and problem-solving.

Diagnosis in the Presence of Other Codes

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Bank 1 and misfire AFRs that are too lean or too rich may indicate an underlying problem that ultimately caused the cat to fail. Sometimes the cat and exhaust are still in good working order. Before worrying about code P0420, resolve these issues. P0420 may be resolved naturally by addressing these underlying issues. At the very least, you want to ensure that the problems do not reoccur.

Related : The Symptoms of a Faulty Fuel Filter

P0420 Troubleshooting/Repairs

We won’t spend much time on this topic because the fix and repairs can vary depending on the diagnosis. Again, replacing the catalytic converters is frequently the solution. Exhaust leaks can be easily repaired with welding. However, if the exhaust appears to be in poor condition overall, this can be a temporary solution. Manifold leaks can also be more complicated on some engines because they are difficult to remove and replace.

O2 sensors and other similar air-fuel sensors can also be the final repair. This may be the best case scenario because sensors are typically less expensive and easier to replace than a full exhaust system or cats.

DTC P0420 Repair Price

How much does it cost to repair engine fault code P0420? The following is a breakdown of repair costs for some common repairs:

  • Catalytic converters cost between $200 and $2,000 or more.
  • O2 Sensors cost between $50 and $300.
  • $0-200 for an exhaust leak (weld).
  • Exhaust leak (replacement exhaust): $200-500+

Catalytic converters are sophisticated emissions equipment that employs high-priced materials. Unfortunately, cat replacement is not an inexpensive job. If you source used converters, the parts alone can cost up to $200. Using new OEM parts can cost more than $2,000 on some engines. This is especially true if several catalytic converters need to be replaced.

On some vehicles, a new O2 sensor can cost as little as $50. However, for most models, $75-150 is probably a more realistic price range. They’re usually simple to replace, but labor can take an hour or two.

Finally, P0420 exhaust leak repairs can vary greatly depending on the solution chosen. If you have the right tools and experience, welding can be free (or very close to it). Welding the holes will most likely cost between $100 and $200 at a repair shop. If you go with new parts, a new exhaust can quickly cost $500 or more.

FAQs about Engine Fault Code P0420

With the bulk of the information out of the way, we’ll go over a few frequently asked questions about code P0420. Some of this may be redundant with the information covered in the preceding article.

The Engine Fault Code P0420  Symptoms - Causes and Solutions

What Does P0420 Stand For?

Catalyst system efficiency is below the threshold, according to the engine diagnostic trouble code (bank 1).

Can I drive with the code P0420?

Yes, driving with P0420 is safe in most situations. It is unlikely to cause any additional issues or concerns about reliability. However, use your discretion and err on the side of caution.

What Are the Consequences of Driving With It?

An exhaust leak or a failing catalytic converter should not pose any significant dangers. Our main concern would be other underlying issues that could result in code P0420. This includes things like lean air-fuel ratios, which can cause severe engine damage.

How Much Does DTC P0420 Repair Cost?

The cost of repairing DTC P0420 varies greatly depending on the diagnosis. If the problem is a simple exhaust leak, you might be able to weld the holes yourself for next to nothing. Catalytic converters, on the other hand, can be costly. In essence, P0420 repairs can cost anywhere from $0 to $2,000 or more.

How Do I Repair P0420 Exhaust Leaks?

P0420 is a common error code caused by exhaust leaks. Examine the exhaust system for obvious damage. Welding could be a simple and inexpensive solution. However, if your exhaust is in poor overall condition, it will most likely only be a temporary fix. It could come down to the need for a new exhaust system, exhaust manifold, and so on.

How Much Do Catalytic Converters Cost?

Yes, they can be quite pricey. Some common metals found inside catalytic converters include platinum, palladium, and rhodium. Those are all expensive materials that lead to an expensive catalytic converter. New ones can cost anywhere from $500 to $2,000 or more. Used options can be found for as little as a couple hundred dollars, but be wary of potentially faulty parts.