Engine Fault Code P0420 – Symptoms, Causes, Fix

Code P0420 is without question one of the most common car engine diagnostic trouble codes (DTC). Some also refer to DTC as engine codes, fault codes, troubleshooting codes, etc. Regardless of the specific words one thing is certain – something is wrong. Worry not, though. In this article, we’ll discuss the meaning, symptoms, causes, and fixes for fault code P0420.

*P0420 is a common code for many car brands including Honda, Toyota, Nissan, Chevy, Ford, Dodge, BMW, etc. If this guide is helpful please feel free to leave a comment. We may consider writing P0420 articles tailored to specific engines or brands if readers find this info useful.

DTC P0420 Meaning

Diagnostic trouble code P0420 will state, “Catalyst system efficiency below threshold (bank 1).”

This points to the engines catalytic converter(s) which are responsible for breaking down exhaust pollutants. Catalysts, or cats, are an important part of an engines emissions system to reduce the harmful gases. DTC P0420 essentially means the catalytic converter on bank 1 is not fully functioning.

However, it’s not always the catalytic converter that isn’t functioning. There are numerous other causes that may trigger fault P0420. The code does mean something with the catalyst/emissions system is wrong, though.

Fault Code P0420 Symptoms

A few symptoms of P0420 catalyst system efficiency below threshold include:

• Check engine light (CEL, MIL, etc)
• Raw exhaust smell
• Poor fuel economy
• Misfires
• AFR lean or rich

Scanning the engines computer via the OBDII port and seeing the code pop up will be the most obvious give-away. It’s common for DTC P0420 to throw a check engine light. Exhaust leaks or poorly functioning catalytic converters may result in a raw exhaust smell. This is similar to rotten egg or sulfur smells. These are the most common symptoms with P0420. In some cases you may notice poor fuel economy.

Misfires, air-fuel ratios too lean or rich, power loss, and misc. symptoms may also occur. However, these are typically symptoms of another underlying issue that’s potentially causing DTC P0420. Anyways, we’ll circle back to this point in the causes of fault code P0420 section.

How Serious Is DTC P0420?

Engine fault P0420 isn’t usually an urgent problem. The issue is unlikely to cause any severe drivability or reliability problems. However, your engine may be releasing more harmful emissions due to the catalyst system operating below efficiency on bank 1. For this reason along with the check engine light you likely won’t pass emissions inspections.

As with any engine problem, the safest option is diagnosing and fixing the P0420 problem as soon as possible. So, we’re not saying ignore these issues for weeks or months. Rather, it’s likely OK to continue driving with engine code P0420. Use your own judgement. If the car feels dangerous or there are other engine codes, serious symptoms, etc then play it safe.

For example, if you notice the engine is running lean it’s best to not drive the engine or at least keep throttle input and RPM’s low. Lean engine operation can potentially put the engine at risk of more serious problems. Also, driving with low power and other drivability issues can pose safety risks.

Engine P0420 Causes

The common causes and reasons for engine DTC P0420 are as follows:

• Worn/failing catalytic converters
• Lack of cats
• Exhaust leaks

Again, the P0420 code means the bank 1 catalyst system efficiency is below the threshold. Either something is physically causing the system to not function or another issue is making the engine believe there’s an issue with the catalytic converter(s).

Worn, failing, or missing catalytic converters are usually the most common cause of DTC P0420. It’s not uncommon for them to become less effective with age, especially after 10+ years and 120,000+ miles. Catalytic converter theft is, unfortunately, a common thing so you could be missing the cats all together.

Exhaust leaks are another common reason to get the fault code. The exhaust system can rust and develop holes. Exhaust manifold gaskets or manifold cracks also occur sometimes.

Otherwise, look for fault O2 sensors (or another similar sensor). The cats may be in good working order, but a bad O2 sensor can make the car believe something is wrong with the cats. Misfires and rich AFR’s can be underlying issues that actually result in catalytic converter failure.

Less Common Causes

Below are a few less common causes of engine fault code P0420:

• Faulty O2 sensor
• Misfires
• Rich AFR’s

Catalysts may actually be in perfect working order. A faulty O2 sensor can lead the engine to believe there’s an issue with the cat and ultimately result in P0420 displaying. Misfires and rich air-fuel rations aren’t often a direct cause of the code. Rather, these can be underlying issues that ultimately lead to the failure of cats. That’s because it allows excess fuel into the exhaust which can then eat away at the catalysts and cause premature failure.

Diagnosing Code P0420

When diagnosing any engine problem it’s important to get the full picture. If there are other diagnostic trouble codes present they may relate to code P0420. We’ll circle back to that in a moment. Here’s a quick breakdown of troubleshooting and diagnosing the P0420 engine code:

1. Ensure P0420 is the only engine code present
2. Clear code and drive for about 15 minutes. Check to see if the code comes back. If not, continue this process a few times to confirm. If DTC P0420 comes back then there’s an actual issue at hand
3. Physically inspect exhaust system for any damage, holes, rust, or other potential leaks. Don’t forget the exhaust manifold as that’s a common area for leaks, too.
4. Check O2 sensor voltage reading. Standard readings should be around 0.4 to 0.5 volts on most engines. If it’s wildly off or bouncing around then it’s likely an issue with the catalytic converter

Sometimes engines have minor hiccups and fault codes randomly appear. In these cases simply clearing and deleting the code may fix the problem. Otherwise, it’s time to track down what’s actually causing DTC P0420. The above steps are pretty effective for exhaust leaks and cat failure. However, if you’re getting other engine codes then it’s going to take a bit more ingenuity and problem solving.

Diagnosing With Other Codes Present

It would be impossible to cover diagnostics for every single engine code that may pop up alongside P0420. Normally it’s fairly straight-forward but requires a bit more thought and research. What other engine code(s) is/are present with DTC P0420? Do they relate to any emissions, exhaust, air-fuel, or other similar systems?

Misfire and bank 1 AFR’s too lean or too rich may point to an underlying issue that ultimately caused the cat to fail. Sometimes the cat and exhaust might still be in good working condition. Diagnose and repair these issues before worrying about code P0420. Fixing these underlying problems may solve P0420 naturally. At the least you want to ensure the problems won’t occur again.

P0420 Fix/Repairs

We won’t spend long on this specific topic since the fix and repairs can vary based on the diagnosis. Again, the fix often comes down to replacing the catalytic converters. Exhaust leaks can be a simple repair with some welding. However, that can be a short-term solution if the exhaust looks to be in poor condition overall. Manifold leaks can also be a bit more complex on some engines since it can be tough to remove and replace.

O2 sensors and other similar air-fuel sensors can be the ultimate repair, too. This may be best case as sensors are typically less expensive and easy to replace compared to a full exhaust system or cats.

DTC P0420 Repair Cost

What are the repair costs for engine fault code P0420? Below is a breakdown of repair costs for some common fixes:

• Catalytic converter: $200-2,000+
• O2 Sensors: $50-300
• Exhaust leak (weld): $0-200
• Exhaust leak (new exhaust): $200-500+

Catalytic converters are complex emissions equipment that use some expensive materials. Unfortunately, that means cat replacement is not a cheap job. The parts alone can run $200 if you source used converters. Going with new OEM parts can exceed $2,000 on some engines. This is especially true if multiple catalytic converters require replacement.

An O2 sensor can run as low as $50 new on some cars. However, $75-150 is probably a more realistic number for most models. They’re generally easy to replace but labor can be another hour or two.

Lastly, P0420 exhaust leak repairs can vary a lot depending on the fix you decide on. Welding can be free (or very close to it) if you have the tools and experience. At a repair shop it will probably be about $100-200 to weld the holes. Opting for a new exhaust can quickly add up to $500+ if you’re going with new parts.

Engine Fault Code P0420 FAQ’s

With the bulk of the info out of the way we will cover a few frequent questions relating to code P0420. Some of this may be duplicate to the information we covered in the article above.

What Does P0420 Mean?

Engine diagnostic trouble code means: Catalyst system efficiency below threshold (bank 1).

Can I Drive With Code P0420 Present?

Yes, in most situations it’s safe to drive with P0420 present. It’s unlikely to cause any additional problems or reliability concerns. Use your judgement and error on the side of caution, though.

What Are the Risks of Driving With It?

An exhaust leak or failing catalytic converter shouldn’t pose any serious risks. Our main concern would be other underlying problems that can trigger code P0420. This includes things like lean air-fuel ratios as running an engine lean can cause severe damage.

How Much Does it Cost to Fix DTC P0420?

The cost to repair DTC P0420 can vary significantly depending on the diagnosis. If it’s a simple exhaust leak you may be able to weld the holes yourself for next to nothing. Catalytic converters can be expensive, though. In essence, P0420 repairs can range from about $0 to $2,000+.

How to Fix P0420 Exhaust Leaks?

P0420 is a common code triggered by exhaust leaks. Inspect the exhaust system for any obvious damage. Welding may be a simple, cheap solution. However, if your exhaust is in poor overall shape it’s likely to only be a short-term fix. It may boil down to needing a new exhaust system, exhaust manifold, etc.

Are Catalytic Converters Expensive?

Yes, they can be very expensive. Platinum, palladium, and rhodium are a few common metals found inside catalytic converters. Those are all expensive materials that lead to an expensive catalytic converter. New ones can run as much as $500-2,000 or more. Used options may sometimes be found for only a couple hundred dollars, but be cautious of potentially faulty parts.