OBD-II Trouble Codes

P0420 Code: Catalyst System Efficiency Below Threshold (Bank 1)

Reading Time: 5 minutes

This code indicates that your car’s computer has detected underperformance from the Bank 1 catalytic converter.

What Does the P0420 Code Mean?

A P0420 code indicates an issue with the catalytic converter. A catalytic converter has an oxygen sensor in the front (upstream) and another on its rear (downstream). While the vehicle is running in closed loop mode, the readings of the upstream Oxygen sensor should ideally fluctuate, while the readings of the downstream O2 sensor should be steady.

The catalytic converter is a critical component of your vehicle’s exhaust system. It is designed to break down harmful pollutants such as carbon monoxide and hydrocarbons that are byproducts of internal combustion. Inside the cat are catalysts (such as platinum, palladium, and rhodium) that convert these toxic emissions into a benign state that is less harmful to the environment.

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

Your car’s computer monitors the downstream O2 sensor to determine the efficiency of the catalytic converter. If both 02 sensors produce similar readings (i.e., they’re both switching back and forth), that often means that the catalytic converter is not functioning as needed.

Then, the Check Engine Light is triggered, and you get a P0420 engine code.

Many owners have reported P0420 for the following makes: Honda (especially on a Honda Accord), Chevrolet, Nissan, Ford, and Toyota.

Causes of the P0420 Code

A faulty catalytic converter is the most frequent cause of a P0420 code. Common issues that can cause the P0420 code include:

  • A failed catalytic converter
  • A faulty oxygen sensor
  • Circuit problems, such as damaged wiring and loose connections
  • An exhaust leak
  • Computer issues (e.g., software in need of an update)

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.

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

automotive catalytic converter
Another cause of P0420 code is a faulty catalytic converter.

Common Symptoms of the P0420 Code

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

  • An illuminated or flashing Check Engine Light
  • A failed emissions test

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. The resources below could help you diagnose your vehicle:

How to Fix the P0420 Code

There are multiple reasons why the P0420 code 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.

The P0420 code could be triggered by anything from a bad catalytic converter to a faulty oxygen sensor, so you must do your homework.

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.

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.

car emissions test
Failed emissions test can be a sign of a logged P0420 code.

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.

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.

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.

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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