Engine Fault Code P0420: Symptoms, Causes, Fix

The P0420 trouble code is without a doubt one of the most commonly encountered (DTC). Detected trouble codes may also be referred to as DTC Engine Codes. It doesn’t matter what the words say, something is wrong. But don’t be alarmed. Fault code P0420 will be discussed in detail in this article, including its meaning, symptoms, causes, and solutions.

A lot of car brands, such as Honda, Toyota, Nissan, Chevy, Ford, Dodge, and BMW use the *P0420 trouble code. Please leave a comment if you found this guide useful. If readers find this information useful, we may write P0420 articles specifically for those engines or brands.

DTC P0420 Meaning

Catalyst system efficiency falls below threshold (bank 1),” says the trouble code P0420.

In this case, it’s the engine’s catalytic converters that are doing the dirty work of breaking down the pollutants in the exhaust. In an engine’s emissions control system, catalysts (also referred to as cats) help reduce the amount of harmful gases being emitted. DTC P0420 basically signifies that bank 1’s catalytic converter isn’t working properly.

The catalytic converter, however, isn’t always the problem. Fault P0420 can be caused by a wide range of different things. It does mean that something is wrong with the catalyst/emissions system, however.

Fault Code P0420 Symptoms

Fault Code P0420 Symptoms

The following are some signs that the P0420 catalyst system efficiency is less than the threshold:

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

Most obviously, scanning the engine’s computer with the OBDII port and seeing the code appear will reveal the problem. DTC P0420 frequently causes a check engine light to come on. a raw exhaust odor may be the result of exhaust leaks or faulty catalytic converters.

There are some similarities here with the smell of rotten eggs or sulfur. P0420 is most commonly associated with these symptoms. You may notice poor fuel economy in some cases.

Air-fuel ratios that are too lean or rich, power loss, and other symptoms such as misfires are all possible outcomes of a malfunction. DTC P0420 can be caused by a number of different problems, and these are just the symptoms of one of them. We’ll come back to this point in the P0420 Troubleshooting Guide.

How Serious Is DTC P0420?

The engine fault code P0420 isn’t a life-or-death situation. Because of this, there should be no major issues with driveability or dependability. However, the catalyst system on bank 1 may be operating at a lower efficiency, causing your engine to produce more harmful emissions. You’ll probably fail emissions tests because of this, as well as the check engine light.

The safest option is to diagnose and fix the P0420 problem as soon as possible, as with any engine problem. This does not mean that you should ignore the problems for several weeks or months.

Instead, if you have engine code P0420, you can probably keep driving as normal. Make an informed decision based on your own experience. Do not drive the car if it appears unsafe or if there are other warning signs such as engine codes or serious symptoms.

If you notice the engine is running lean, for example, you should avoid driving it or at the very least limit your throttle input and engine RPMs. Running a lean engine can put your engine at risk of breaking down. In addition, poor maneuverability and other drivability issues can put your life in danger.

Engine P0420 Causes

When engine DTC P0420 appears, the most common causes and reasons are as follows:

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

The P0420 error code indicates that the efficiency of the bank 1 catalyst system is less than the required level. Either the system is physically broken, or the engine thinks the catalytic converter is broken because of some other problem (s).

DTC P0420 is frequently caused by a catalytic converter that is failing, worn out, or missing. Over the course of ten years and 120,000 miles, it’s not uncommon for them to lose some of their functionality. Unfortunately, catalytic converter theft is common, and as a result, you may have completely missed the cats.

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

Search for faulty O2 sensors instead (or another similar sensor). It’s possible that the cats are in perfect working order, but a faulty O2 sensor will lead the car to believe otherwise. Catalytic converter failure can be the result of underlying problems such as misfires and high AFRs.

Less Common Causes

Some of the less common causes of engine fault code P0420 are listed below:

  • Faulty O2 sensor
  • Misfires
  • Rich AFR’s

It’s possible that the catalysts are perfect. This code is displayed when the engine thinks there is a problem with the catalytic converter, which can be caused by a faulty O2 sensor. Not all codes are caused by misfires or excessive air-to-fuel rations.

Instead, these may be underlying problems that contribute to cats’ failure. This is due to the fact that it allows too much fuel to be burned off in the exhaust, which damages the catalysts and shortens their lifespan.

Diagnosing Code P0420

Diagnosing Code P0420

It’s critical to look at the big picture when trying to figure out what’s wrong with an engine. The code P0420 may be linked to other DTCs present in the system. Later on, we’ll return to this topic. P0420 Engine Code Troubleshooting and Diagnosis: Here’s a Quick Guide

  • Check to see if P0420 is the only error code displayed on the dashboard.
  • Drive for about 15 minutes, then clear the code on your phone. See if the code is returned. If this doesn’t work, repeat the procedure several times until it does. There’s a real problem if DTC P0420 resurfaces.
  • You should physically check your exhaust system to make sure there aren’t any damage such as cracks or holes. Exhaust manifolds are another common location for leaks, so be sure to check there as well.
  • Check the voltage on the O2 sensor. Most engines have a voltage range of 0.4 to 0.5 volts as a standard. It’s possible that the catalytic converter is malfunctioning if the engine is wildly off or bouncing around.

Fault codes can appear out of nowhere when engines have minor hiccups. The problem may be resolved simply by deleting and clearing the code in these circumstances. Unless something has changed, it’s time to determine what’s causing DTC P0420.

Exhaust leaks and catalytic converter failure can both be remedied with the steps listed above. Nevertheless, if you’re getting other error codes, you’ll have to put in a little more effort and think outside the box.

Diagnosing With Other Codes Present

To cover diagnostics for every possible engine code alongside P0420 would be impossible. In most cases, it’s simple, but it necessitates some additional thought and research. DTC P0420 also has the following engine code(s) associated with it: Can you tell me if this has anything to do with any kind of emission or exhaust system?

Bank 1 and a misfire If the AFR is too low or too high, it could indicate a problem that ultimately led to the cat’s demise. Catalytic converters and exhaust systems can sometimes be found to be in good working order.

Prior to focusing on error code P0420, look for and fix these other problems. Fixing the underlying issues may help P0420 go away on its own. At the very least, you want to make certain that the issues will not recur.

P0420 Fix/Repairs

We won’t spend a lot of time on this particular subject because the fix and repairs may differ depending on the diagnostic. In many cases, the solution is as simple as swapping out the catalytic converters. Some welding can fix exhaust leaks quickly.

However, if the exhaust appears to be in poor overall condition, this may be a short-term solution. The removal and replacement of the manifold on some engines can be difficult, making manifold leaks even more difficult to diagnose.

Other air-fuel sensors, such as O2 sensors, can be the final repair. This is the best-case scenario because sensors are less expensive and easier to replace than a complete exhaust system or catalytic converters.

DTC P0420 Repair Cost

How much does it cost to fix a P0420 engine code? The following table breaks down the costs of common repairs:

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

A catalytic converter is an expensive piece of pollution control equipment. As a result, cat replacement is an expensive proposition. If you buy used converters, the parts alone will cost you $200. On some engines, using new OEM parts can cost well over $2,000 instead of reusing old ones. This is especially true if you need to replace more than one catalytic converter.

Some automobiles’ O2 sensors cost as little as $50 brand new, according to the manufacturer’s list price. However, for most models, $75-150 is a more reasonable price range. While they’re relatively simple to replace, the labor involved can add an additional hour or two to the project.

Finally, the cost of fixing a P0420 exhaust leak varies widely depending on the solution you choose. If you have the right equipment and knowledge, welding can be completely free. Welding the holes will cost between $100 and $200 at a repair shop. If you’re using new parts, getting a new exhaust system can cost upwards of $500.

Engine Fault Code P0420 FAQ’s

In this section, we’ll address some frequently asked questions about error code P0420. Some of it may be redundant with what we’ve already discussed.

What Does P0420 Mean?

Catalyst system efficiency is below the threshold, according to this error code in the engine management system (bank 1).

Can I Drive With Code P0420 Present?

Yes, you can drive with P0420 on most of the time. It’s unlikely to cause any new issues or raise any new questions about its dependability. However, exercise caution and good judgment.

What Are the Risks of Driving With It?

Even if you have an exhaust leak or a failing catalytic converter, you should be safe. Code P0420 can be caused by a number of different issues, but our primary concern is with any underlying issues. Running an engine lean can cause serious damage, and that includes things like lean air-to-fuel ratios.

How Much Does it Cost to Fix DTC P0420?

The price of DTC P0420 repair varies widely based on the diagnosis. If the problem is an exhaust leak, you may be able to fix it on your own for very little money by welding the holes yourself. On the other hand, catalytic converters can be quite costly. Repairs for P0420 systems can cost anywhere from zero dollars up to several thousand dollars.

How to Fix P0420 Exhaust Leaks?

Error code P0420 is frequently associated with exhaust leaks (P0420). Look for obvious damage to the exhaust system. Welding could be a low-cost and simple option. This will only be a temporary solution if your exhaust is in poor overall condition. A new exhaust system, manifold, or other component may be required.

Are Catalytic Converters Expensive?

They can be quite pricey, yes. Catalytic converters often contain platinum, palladium, or rhodium as well as other rare metals. All of those components add up to a pricey catalytic converter.

Buying a new one can cost anywhere between $500 and $2,000 depending on the model. For a few hundred dollars or less, you can often find pre-owned options, but keep an eye out for faulty components.

Up Next: Engine Fault Code P0300 – Symptoms, Causes, Fix

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