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There are many issues that could trigger the Check Engine Light to illuminate. However, if you use an OBD-II scanner and get a P0420 code, you may be having issues with your catalytic converter. 

For a full rundown of what this code entails, read on.

What Does the P0420 Code Mean?

Code P0420 stands for “Catalyst System Efficiency Below Threshold (Bank 1).” Put simply, this code indicates that your car’s computer has detected underperformance from the Bank 1 catalytic converter. 

a catalytic converter of a modern car close up view
The P0420 Code indicates that your car’s computer has detected underperformance from the Bank 1 catalytic converter. 

Bank 1 refers to the side of the engine that houses the number one cylinder. The opposite side of the engine is Bank 2. 

What are the Possible Causes of the P0420 Code?

A faulty catalytic converter is the most frequent cause of  a P0420 trouble code. Issues that can cause this OBD-II code include:

  • A failed catalytic converter (very common)
  • A faulty oxygen sensor (not very common)
  • Circuit problems, such as damaged wiring and loose connections (not very common)
  • An exhaust leak (not very common)
  • Computer issues (e.g., software in need of an update) (not very common)

The catalytic converter can also fail due to a number of other problems. These include engine performance issues, such as a misfire or improper air/fuel ratio, which can cause the converter to overheat.

Loose wiring of a car
Damaged wiring and loose connections are possible causes of the P0420 code.

Contaminants like coolant and engine oil can also damage the cat.

What are the Common Symptoms of the P0420 Code?

You may notice the following symptoms if you drive a vehicle with a P0420 trouble code:

P0420 typically means that a converter has worn out or degraded—not obstructed or clogged. But if the case is the latter, it can lead to engine performance problems, such as a lack of power, rough idling, and stalling, which are some of the common signs of a clogged catalytic converter

How to Diagnose the P0420 Code

There are numerous potential causes for OBD-II code P0420. As such, diagnosis can be difficult. For an idea of how to troubleshoot the code, check out the videos below:

How to Fix the P0420 Code

There are multiple reasons why code P0420 might be stored. Therefore, there isn’t a “magic bullet” fix for the issue. You’ll need to diagnose the code accurately, as outlined above, then perform any necessary repairs. Usually the catalyst needs to be replaced, and bolt-on catalysts are available for most vehicles.

The code could be triggered by anything from a bad catalytic converter to a faulty oxygen sensor, so you must do your homework.  

car diagnostic tool for car checkup being used by a mechanic
There isn’t a “magic bullet” fix for the issue. You’ll need to diagnose the code accurately, then perform any necessary repairs.

Also, keep in mind that all vehicles are different. When troubleshooting and repairing diagnostic trouble codes, you should consult the factory repair information for your application. 

Repair manuals, such as those from Chilton, are useful, but an ALLDATA subscription is even better. ALLDATA has single-vehicle subscriptions for DIYers that provide detailed factory repair information. 

What is a Catalyst?

The catalysts on today’s vehicles are “Three Way Catalysts,” meaning that they process three harmful gasses, namely NOx (Oxides of Nitrogen), CO (Carbon Monoxide), and HC (Hydrocarbons, which is unburned fuel)

A “catalyst,” by definition, foments changes in other elements without changing itself – the other elements in this case are the three harmful exhaust gasses.

Inside of a catalytic converter showing a honeycomb design
A catalytic converter has within its shell a fine honeycomb type clay “brick” with thousands of tiny “tunnels” tightly arranged so that the exhaust gas must pass through them as it exits the engine.

To treat these exhaust gasses, a catalytic converter has within its shell a fine honeycomb type clay “brick” with thousands of tiny “tunnels” tightly arranged so that the exhaust gas must pass through them as it exits the engine. The walls of these tiny tunnels are meticulously coated with certain precious metals that, as the catalyst reaches operating temperature, will either separate nitrogen and oxygen or add molecules of oxygen, but you can’t do both jobs with the same “brick.” Sometimes both “bricks” are in the same shell – more often, they’re not.

The catalyst brick nearest the engine takes care of the NOx and is referred to as the “light-off cat.” This catalyst has to reach a certain temperature to begin the catalytic process of separating NOx into its nitrogen and oxygen components. This is the only catalyst that is monitored by the ECM/PCM.  

So why is this process necessary? Well, nitrogen and oxygen naturally bind together in the combustion chamber during high combustion temperatures, such as at road speed and when the vehicle is pulling a load.

In the process of doing its job, the light-off (NOx) catalyst stores oxygen; there is an O2 sensor between the engine and the light-off catalyst, and there is another O2 sensor behind the light-off catalyst but in front of the HC/CO catalyst. 

The second catalyst, which is farthest from the engine, handles HC and CO by adding molecules of oxygen and converting them to CO2, which is actually good for the environment, since grass, trees, and flowers need CO2 to thrive.

car electronic engine control unit close look
When the ECM/PCM sees the downstream O2 sensor switching at or near the same rate as the upstream O2 sensor, it sets the P0420 code.

The O2 sensor between the engine and the light-off catalyst “brick” (that’s the Upstream Sensor) monitors the oxygen level in the exhaust and the ECM/PCM uses that input for Fuel Trim adjustments. 

That Upstream O2 signal switches very rapidly in response to Fuel Trim (several times a second). The O2 sensor behind the light-off catalyst (the Downstream Sensor) is also monitoring oxygen, but if the light-off (NOx) catalyst is storing oxygen properly (meaning the catalyst is healthy), the rear O2 sensor signal will be very lazy when compared to the upstream O2. When the ECM/PCM sees the downstream O2 sensor switching at or near the same rate as the upstream O2 sensor, it sets the P0420 code.

Other Notes About P0420 Code

Make sure that you have ruled out other possible causes of inefficiency in the catalyst system before replacing your O2 sensors or catalytic converter. Some car owners make the mistake of replacing a perfectly fine catalytic converter because of a wiring or software issues. It is best to consult a licensed mechanic to ensure that you get a proper diagnosis so that you don’t waste money on replacing parts unnecessarily.

automotive catalytic converter
Make sure that you have ruled out other possible causes of inefficiency in the catalyst system before replacing your O2 sensors or catalytic converter.

P0420 Code FAQs

How do I clear a P0420 code?

You can clear the P0420 code temporarily with a scan tool or code reader. The problem is, the code will come back as soon as your car’s primary computer runs its system self-tests. To get the code to go off and stay off, you’ll need to fix the underlying problem that triggered the code in the first place, then clear the code with a scan tool or code reader.

Can a bad O2 sensor cause a P0420 code?

Yes. A typical catalyst monitor uses the signal from the downstream oxygen sensor to determine catalyst efficiency, and the signal from the upstream sensor is a reference point. As such, failure of either the upstream or downstream oxygen sensor can cause code P0420 to set. The most common cause of the code, however, is a failed catalytic converter.

an oxygen sensor on a catalyst pipe of gasoline engine car close up
Failure of either the upstream or downstream oxygen sensor can cause code P0420 to set.

What is the p0420 code on a Nissan?

The P0420 code is a generic OBD code with a standard definition from the Society of Automotive Engineers (SAE). SAE refers to the code as “Catalyst System Efficiency Below Threshold (Bank 1)”. But on many newer Nissan vehicles, when using an OEM-level scan tool, the code may come up as “Three-Way Catalyst Malfunction.” In either scenario, the code indicates that your car’s onboard computer perceives a decline in the efficiency of the catalytic converter(s).

How do you test an O2 sensor for a P0420 code?

Most of the time, code P0420 indicates a faulty catalytic converter. There are, however, instances where a bad oxygen sensor can trigger the code.

mechanic conducting diagnostics tests on a vehicle
Monitor the sensors with a scan tool. The sensors should go rich in response to the propane.

One way you can rule out the oxygen sensors is by forcing the air/fuel mixture rich and then lean. To force the system rich, carefully add propane to the engine’s air intake (and raise RPMs so the engine doesn’t stall) to enrich the air/fuel mixture. At the same time, monitor the sensors with a scan tool. The sensors should go rich (about 800 mV to 900 mV for a traditional zirconia oxygen sensor) in response to the propane.

You can then force the system lean by disconnecting a large vacuum hose. In this state, the sensors should go lean (around 200 mV-300 mV) for a traditional zirconia oxygen sensor).

If the oxygen sensors respond properly to both the rich and lean conditions, they’re working as they should, and the catalytic converter is likely the cause of the P0420 code.

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Any information provided on this Website is for informational purposes only and is not intended to replace consultation with a professional mechanic.

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Samuel Snead

I recently had my sister van diagnosed and

p0420 had shown up and I was wondering if I was to get rid of the cat and straight pipe would this create more problems for me or would I be best to replace the cat

Samuel Snead

I recently had put my sister van diagnosed and p0420 had popped up on the diagnosis along with other codes and I was wondering if I get rid of the cat and straight pipe would I have the same problems or should I just replace the cat with another one

sinabasta

Hi, I have an 03 Tundra 3.4l v6, no matter how many miles I drive I can’t seem to set my catalyst monitor for smog check. No engine code, temp on my front cat inlet 350F outlet 520F, the rear cat inlet and outlet are the same at 320F, any idea is appreciated.

Michele Wallick

I have a 2017 Chevy sonic and I’ve sunk about 3 grand into it to get rid of this code. And it still won’t go away.

Kita Davis

I had a P0420 code pop up an my Mechanic put a cleaner in it my gas tank an it went off however I do need a fuel pump do u think that part of P0420? And why it popped up?

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