Engine Code p0420 Meaning, Causes, Fixes (3 Steps)

Is your car currently plagued by a persistent check engine light, and a p0420 fault code?

If so, you're in the right place! 

In this PopularMechanic article you will learn:

  • What Does The p0420 Code Mean?
  • What Caused The p0420 code (Possible Symptoms)?
  • How Do You Diagnose A p0420 Code?

And much more!

Engine Code p0420 meaning, causes, fixes (3-steps)

Although it might seem as if your ride has left you out to dry, the occurrence of a p0420 fault code is more common than one might think, especially in vehicles that have spent a fair number of years on the road.

Luckily, once diagnosed, the dreaded p0420 code is seldom difficult to remedy.

In fact, a single afternoon of time behind the wrench, and your car will be running as good as ever, once again ready to handle anything that the road can throw its way.

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What Does The p0420 Code Mean?

Fault Code p0420 is defined as “catalyst system efficiency below threshold- bank 1.”

In everyday terms, this code is stating that a less than ideal amount of exhaust flow is being allowed to pass through a vehicle's catalytic convertor, leading to poor catalyst efficiency. 

This code is historically at fault for many failed emissions tests, in states where such testing is mandated.

A vehicle’s catalytic converter is responsible for reducing the harmful pollutants found in a car, truck, or SUV’s exhaust output. An error code p0420 means that a vehicle can no longer do this efficiently.

What Caused The p0420 code (Possible Symptoms)?

A vehicle's exhaust system features at least two O2 sensors. These sensors monitor the amount of oxygen in post-combustion exhaust gasses as they travel through this system.

One of these downstream oxygen sensors can be found just before the catalytic converter, while the other is found directly after this same converter.

Values from these sensors are relayed back to a vehicle’s engine management computer, or PCM, where they are factored against additional forms of input data.

When values from the second of these two sensors drift from their typical range, a vehicle’s ECM (Engine Control Module) will store a p0420 trouble code, illuminating a check engine light in the process.

The root cause of the p0420 code can vary, depending upon the vehicle and situation at hand.

A faulty catalytic converter is one common cause of a p0420 code, though exhaust leaks, misfires, or a defective oxygen sensor are also a suspect.

Some common symptoms often associated with the presence of a p0420 fault code are a lack of overall engine performance and acceleration, difficulty achieving speeds that were previously easily reached, and the sulfur-like, rotten egg smell.

Check out this article for more information on an error code p0420 and a faulty catalytic converter.

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How To Diagnose A p0420 Code?

  1. Check for Additional Codes
  2. Perform a Visual Inspection
  3. Check O2 Sensor Values

To make the needed repairs in order to eliminate a p0420 fault code, you must first diagnose the root cause of your vehicle’s issue.

A thorough diagnosis assists you in making quick and concise repairs and eliminates excess expenditure on unneeded parts.

The following are the steps to diagnosing your car, truck, or SUV’s p0420 fault code. 

Step 1: Check For Additional Codes

Before proceeding further in your diagnosis, you should verify that p0420 is the only active powertrain fault code present.

This can be easily done with any level of OBD II scan tool.

The presence of other codes, such as those those related to can fuel pressure or your vehicle’s fuel injectors, tip you off to other issues that can be triggering a p0420 code as a secondary code.

If any misfire, lean, or rich codes are discovered, they should be pursued before further diagnosis.

Step 2: Perform A Visual Inspection

If p0420 has been isolated as the only fault code present, the next course of action is to visually inspect all exhaust components, including the catalytic convertor.

Look for any signs of cracks in your vehicle’s exhaust pipes, loose clamps, or broken exhaust manifold studs/bolts.

Additionally, examine O2 sensor wiring for fraying or loose connections. If any issues are found, make needed repairs before proceeding.

Step 3: Check O2 Sensor Values

With the use of a quality OBD II scanner and multimeter, you will now carefully study the 02 sensor values.

You should notice rapid switching of the pre-converter, upstream O2 sensor at operating temperatures, while downstream O2 readings should settle in at approximately .45 volts during closed loop operation.

If this voltage reading continues to constantly fluctuate, a catalytic converter malfunction is suspect. This article explains more about O2 sensors.

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How Do I Fix An p0420 Code?

  1. Source All Parts
  2. Carefully Replace Defective Components
  3. Verify Repair

With a firm diagnosis in hand, it will be time to make any needed repairs.

For any questions related to the exact replacement procedure of components for your vehicle, always consult model specific service literature. 

Step 1: Source All Parts

Before disassembly always take the time to locate and purchase all needed parts in advance.

While any issue resulting in a p0420 code is certainly cause for immediate attention, continued vehicle operation is generally acceptable for a short period of time until a repair can be made.

Therefore, it is not advisable to begin old component removal until you have your new parts in hand, as this prolongs vehicle downtime.

Step 2: Carefully Replace Defective Components

By this point, you have likely isolated the cause of your vehicle’s p0420 diagnostic trouble code to a worn catalytic converter, or a faulty oxygen sensor.

Both of these parts are located within the exhaust system, making them prime candidates for difficult removal.

Spray all clamps, fasteners, and exposed threads with a quality penetrating oil, and carefully free the defective component from the vehicle. Your new converter or O2 sensor will be installed in the reverse fashion of removal.

Ensure that every item is torqued to specification. It can also prove helpful to only use OEM grade parts and avoid the use of aftermarket products when possible.

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Step 3: Verify Repair

With the installation of your new catalytic converter or O2 sensor complete, clear the previously notated p0420 error code, and test drive your vehicle.

Be sure to make several short trips, as this gives a vehicle’s monitors time to perform an adequate number of self-tests to verify that no further problem exists.

On-Board Diagnostic systems (OBD and OBD-2) were only made standard in 1996. Every car from that point onward used an OBD system for easier fixes.

Taking Repairs Into Your Own Hands

While the discovery of a p0420 OBD2 DTC never comes as welcome news, those who are patient, attentive, and comfortable with basic car repair can make quick work out of repairing an issue of this nature.

If the time is taken to identify and further diagnose the issue at hand, with a little resolve, you will be well on your way to having your ride restored to its former level of reliability.

About Your Mechanic

About Josh Boyd

I’ve been an ASE Certified automotive/heavy equipment technician since 2010. I began my career performing basic automotive maintenance at a service center before working as a Toyota technician at my local dealership.

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