P0420 Error Code – Meaning, Symptoms, and How To Fix It


Have you recently scanned your vehicle with a diagnostic tool and a P0420 error code came up? Are you wondering what can trigger a P0420 error code, what does it mean, and how to fix it?

I have had a lot of people coming to me with the P0420 error code. This error code is pretty common, especially on vehicles with over 100k miles.

So, I cleared my afternoon schedule and gave myself the task to write about the P0420 error code and everything you will need to know about it.

What Does P0420 Error Code Mean?

P0420 Error Code - Meaning, Symptoms, and How To Fix It

What does P0420 error code mean? The “P0420” error code is the generic OBD-II trouble code for catalytic converter damage. P0420 code (Catalyst System Efficiency Below Threshold – Bank 1) can be caused by excessive fuel pressure, low fuel pressure, evaporative emissions system leaks, exhaust leaks, or faulty EGR or pre-catalytic converters.

A vehicle’s catalytic converter has three primary jobs: reduce toxic emissions, provide more power by increasing fuel efficiency, and save energy across the board.

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Catalytic converters are fairly complex but not delicate. They can run for years without requiring replacement or repairs. This is why it is so hard to pinpoint what actually caused this error code to return.

P0420 and other emission-related codes do not mean that something is actually wrong with the vehicle. They are simply a flag for repair technicians to watch certain components or systems that may need attention at some point in the future.

P0420 may mean you can pass smog now. However, it could signal an issue with your engine later if this problem is not corrected.

When you clear the error code without fixing the problem, it will most likely come back. So, don’t delay fixing these issues until they turn into critical problems resulting in unpredictable driving conditions and major damage to your car.

If your P0420 code appears and disappears and/or if the accompanying check engine light turns off and on while driving, then it may be accompanied by other errors which you should also have diagnosed as soon as possible.

It’s not always necessary to fix all these other problems right away. But, you need to be aware that there are several different components causing this issue. So, address them each accordingly until the check engine light returns no more.

What Does The Catalytic Converter Do?

What does the catalytic converter do? A catalytic converter is a device used on the exhaust system of gasoline and diesel engines to reduce harmful pollutants. Without catalytic converters, our air could be unfit to breathe due to excess pollution caused by vehicles.

Coating exhaust valves, manifolds, pipes, etc. with noble metals such as platinum increases their surface area and thus enhances the chemical reactions required for this reduction by several orders of magnitude to make it possible.

How Do The Catalytic Converters Work?

16 Causes and Fixes For Error Code P0420 In Cars

Catalytic converters accomplish their task by means of a “three-way reaction” also known as the “catalytic cycle” or just the “catalytic reaction”.

Catalytic converters have to be made from refractory materials able to withstand high temperatures made from exhaust gases. Platinum’s resistance at elevated temperatures makes it ideal for this purpose. The platinum is applied as a thin film to the walls of the converter.

Catalytic converters have two chambers: an upper one where oxidation takes place and a lower one where reduction takes place. The exhaust gases enter through a pipe inlet near the bottom of this lower chamber. They then flow up through an annular space that is open at the top. As they rise upward, some of them reach the hot surface of the catalyst and release NO.

The oxygen required for this last step comes from air entering the converter through a second inlet pipe located at the top of the upper chamber.

Other metals can be used to replace platinum if need be for economic reasons—for example, rhodium, palladium, or even iron. However, their chemical activity does not equal that of platinum’s by several orders of magnitude which reduces drastically the effectiveness of their use.

No device is perfect and catalytic converters are certainly no exception. They produce some pollution themselves!

Gasoline engines do indeed operate with a lean mixture (excess of oxygen). This means that some nitric oxide will escape unburned. Moreover, the NO reacts with atmospheric moisture to produce nitrogen dioxide, another pollutant gas.

Developments in recent years have considerably reduced these losses. The overall efficiency of the catalytic converter has increased owing to several design improvements such as using precious metals alloyed with other elements like rhodium or palladium, applied on special carrier materials for better dispersion and improved mechanical strength, etc.

Today’s vehicles are equipped not only with one but often with two (or even three) catalytic converters.

Three types of catalytic converters:

  1. One pre-catalytic converter upstream of the main (after-) catalytic converter where most pollutants are actually produced.
  2. An intermediate converter that neutralizes the pollutants from the first one before they can reach and poison the main catalytic converter.
  3. A post-catalytic converter where gaseous emissions are reduced yet another time. This limits their release into the atmosphere even more.

The three converters may be designed as entirely separate structures or as a single piece of equipment with two chambers separated by a wall allowing passage of exhaust gases from top to bottom. They are usually made from honeycomb structure to save space.

The latest diesel engines use more sophisticated technologies for reducing their pollution which does not involve catalytic converters at all:

  • Exhaust Gas Recirculation (EGR) recirculates some of the engine’s exhaust gas back into its own inlet system. It reduces the temperature of the exhaust gases to avoid damage to other components. Also, it dilutes their oxygen content. This reduces combustion temperatures, thus reducing NO x formation while also increasing power output by up to 3%.
  • Selective Catalytic Reduction (SCR) uses an extra injection of liquid urea near the main catalytic converter. This causes chemical reactions leading to reduced NO x levels in exhaust gases.
  • Diesel Particulate Filter (DPF) traps soot particles generated by incomplete combustion reducing particulate matter emissions massively!

16 Causes and Fixes For Error Code P0420 In Cars:

16 Causes and Fixes For Error Code P0420 In Cars

There are a lot of malfunctions that can trigger the P0420 code. Below, I will put together a list of 16 causes and fixes for the P0420 code in cars.

1. Exhaust leak before the catalytic converter.

An exhaust leak before the catalytic converter means that there could be a leak in the muffler, exhaust manifold, or exhaust pipe.

Solution: In order to fix the exhaust leak before the catalytic converter, you will need to replace the damaged exhaust components.

2. High fuel pressure

Check if there are any loose connections between the intake manifold and engine valve cover gaskets.

Solution: If you have a high mileage car then replace the high-pressure fuel pump, fuel filter, or both.

If your car is running rich then it may very well be caused by incorrect fuel pressure. In some cases, the check engine light can even come on as a result of this issue.

3. Low fuel pressure

Refer to the list of the possible causes above. Check the condition of the fuel pump relay and wiring between it and the high-pressure pump.

Solution: If everything looks fine then you may have to replace the high-pressure pump.

4. Exhaust leaks

Exhaust leaks is pretty much self-explanatory issue. It means that somewhere along the exhaust, there is a leak. This can be due to rust, tear, or a bad welding job.

Solution: Fix exhaust leaks with butyl rope or another sealant. Replace leaking flexible hoses or metallic pipes that have developed cracks over time.

5. EGR system failure

Test for EGR blockage first by starting your car in open-loop mode(ECU switching to open-loop). This means no oxygen sensor feedback to adjust the air/fuel mixture ratio on your engine. So, make sure you’re driving on an open road. If the car idles fine then you likely have a clogged EGR valve or stuck EGR solenoid.

Solution: Replace or clean the EGR system.

See: How To Clean an EGR Valve Without Removing It

6. Pre-catalytic converter oxygen sensor failure

Exhaust leaks can make your ECU think that there’s no pre-cat left so it switches to fail-safe mode and throws P0420 error code along with P0171 (lean exhaust gas mixture).

Solution: If this is the case then you need to first fix exhaust leaks, replace the problem component(s), clear code, and see if it comes back.

7. PCM internal failure

Look for other codes before making this conclusion as there are many other possible causes of multiple trouble codes. Also, make sure there are no loose or broken wires before assuming this as a possible cause.

8. Burned exhaust valves

These can be a result of a rich fuel mixture or oil contamination from internal engine parts. If your car only has the P0420 trouble code, then you’re most likely looking at leaking/bad valve seals. It’s rare that all 4 valves would go bad at once.

In rare cases, burnt valves can also happen for other reasons such as lack of maintenance & old age.

9. Catalytic converter damage due to misfiring engine

This is not very common, but it does happen. To fix this you will first need to replace the carbon deposits on the front of your catalytic converter(s) with steel wool or fine wire brush, and finally replace damaged converter(s).

10. Cracked exhaust manifolds

The crack will usually start near where the manifold meets up with the head and will go down towards the converter.

If you’re lucky, it’ll be near enough to the flange where you can cut off that section of piping and replace only that section instead of the entire manifold (which requires more work).

Replacing manifolds is not cheap but there are used manifolds available online which make it an affordable option for DIY’ers.

11. Internal engine damage due to bad rings or valves

Weak compression, low oil pressure, oil leakage from valve cover gaskets, etc. This is very unusual if your car runs fine otherwise. So, keep this in mind if everything else has been checked & replaced as normal wear parts. In most cases, these damages are caused by a lack of maintenance.

12. PCM failure

This is unlikely because the ECU will throw a variety of misfire/oxygen sensor-related codes before just giving up and throwing the P0420 code.

In rare cases, the ECU may have gotten fooled by unusual readings from one or more sensors as a result of carbon buildup on wiring harness connectors. So, check those first before going for this option.

Also, keep in mind that some O2 sensors produce very weird readings under certain conditions. So, trying to read error codes & do voltage checks with these particular sensors may not be very reliable.

13. Bad or missing EGR valve

Cleaning or replacing the EGR valve is not easy so if possible I would recommend just getting a new one. Usually, it’s best to get the exact replacement because you never know what goes on prior to your purchase & they’re not exactly cheap either.

Solution: If you are unable to replace it then I’d look into having someone do it for you. The replacement shouldn’t be too expensive but even then, something could go wrong later down the road…

14. Bad EGR solenoid (part of the EGR valve assembly). The

EGR solenoid is an electronically-controlled valve that, under certain engine conditions (full engine load and/or high engine temperature), opens and allows a small amount of exhaust gas to be routed back into the intake manifold.

Solution: If your EGR solenoid is bad, it will throw a P0420 code. Replacing the EGR solenoid will fix this problem.

15. Bad PCV valve or incorrect PCV hose routing

Confirm that your PCV vent hose is routed to the intake manifold rather than an aftermarket catch can. If you have any other type of catch can installed, then remove it & reroute the hose back to the intake manifold.

Also ensure that you’re using an OEM PCV valve because some B-series intakes are designed specifically for one or the other so if you get a generic one instead, performance will suffer greatly.

16. Leaking intake manifold gasket

Like carbon buildup on wiring harness connectors, this is generally caused by long-term neglect or older vehicles. Especially if there’s any coolant or oil residue found on top of the intake.

If it’s not leaking and you’ve cleaned up all of the residues then it may very well be fine. But, I do recommend replacing stock gaskets as preventive maintenance every 10-15 years (depends on mileage).

Is it safe to drive with an error code P0420?

The answer to this question lies in your owner’s manual. If your owner’s manual says that an error code P0420 should be fixed immediately, then it is not safe to drive with an error code P0420. However, if the owner’s manual says to ”monitor system”, then you can still continue to drive your vehicle and eventually get it repaired.

This is the most frequently asked question, and everyone wants to know if it’s safe to drive with a P0420 code on their car. The answer will depend on many factors.

If the check engine light (CEL) isn’t on, you can drive normally with a P0420 – especially if you don’t notice any other issue than the CEL itself.

These days many cars are equipped with two or three catalytic converters on each bank of cylinders. If only one converter is causing the issue, you can still drive normally with the remaining cat converters.

There is a certain drawback to driving like that though. Since there are two catalytic converters on each side of your engine block, both should be equally dirty or equally working at any given time.

Driving with half-working catalytic converters will cause severe damage to them in less than 100k miles. A completely broken catalytic converter can set you back a few thousand bucks (for the part alone). When your car fails smog inspection, for example, this is usually the reason why.

One more thing to consider is that newer cars also have two O2 sensors on each bank of cylinders. One before and one after the catalytic converter. So, driving like that can cause your car to report an error in either or both O2 sensors since they are no longer receiving correct signals from both catalytic converters on each side of your engine block.

How To Diagnose The P0420 Error Code?

The P0420 is one of the most common OBD II trouble codes. Any time the Check Engine Light illuminates, this code should be highly suspected. This article will guide you through how to diagnose your vehicle’s MIL (Malfunction Indicator Lamp) and determine if a P0420 error exists for any given vehicle.

A P0420 means that there is a fault in the catalytic converter and emissions control system which restricts/blocks it from proper emission testing and regulating gas mileage accurately.

There may also be other causes such as:

  • A defective oxygen sensor or wiring issue with the oxygen sensor
  • Faulty engine components such as wires coils, plugs, etc…
  • Malfunctioning ECM (Engine Control Module)
  • Low fuel pressure or a weak fuel pump

Steps to diagnose the P0420 error code:

  1. A simple way to check if the problem is in the catalytic converter.
  2. Disconnect one of the oxygen sensors located before and behind it. You need an diagnostic tool to check if the error code is reset from your vehicle’s computer. If so, then verify that your ignition coils are firing properly by revving the engine.
  3. If there aren’t any misfires but the Check Engine Light remains on, it is likely not going to be a defective oxygen sensor or wiring issue with it.
  4. You may have a bad catalytic converter or other engine components.
  5. Another common method used to diagnose P0420 problems is to inspect the wiring/sensor harnesses and oxygen sensor for any signs of damage or severe wear. This isn’t necessary unless you suspect that the alternator belt just came off and likely fried your wiring harness if it did come off, which is an issue many people face when this code pops up on their vehicle. It’s better to be safe than sorry in such a situation. So, take a good look at these before proceeding with diagnostics.

How Much Will It Cost To Fix A P0420 Code?

On average, expect to pay anywhere from $80 – $400 in order to get a P0420 code repaired. This price will differ based on the year, make & model of your vehicle, and how extensive the check is that needs to be done.

Will Catalytic Converter Cleaner Fix P0420?

Catalytic converter cleaner is a product that either attaches to the catalytic converter and blasts it with cleaning agents or sprays liquid cleaners onto the catalyst.

The idea is to eliminate trapped particles by blowing them away or flushing them out with the liquid. And, if done regularly (every four months), a catalytic converter cleaner might fix the P0420 code.

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