Error code P0420 is a diagnostic trouble code or DTC. It is a generic powertrain code. It has been labelled generic code because it can be applied to all models of vehicles from the year 1996 to present, although there are different steps on how to handle this error depending on the model of car. This engine code would also apply to Toyota, Nissan, Ford, Chevrolet, GMC, Honda and many other popular manufacturers.

The error code P0420 is a fairly common code and there are a multiple ways of fixing this error. Repairing error code p0420 is generally inexpensive, so there’s no major cause for concern if it appears.


The P0420 diagnostic trouble code is known as a generic fault code. It is set when the Onboard Diagnostic II or OBD II catalyst monitor can sense a drop in its converter efficiency. The OBD II can monitor catalyst efficiency by comparing the switching activity of the upstream and the downstream oxygen sensors in the exhaust. Behind the catalytic converter or the downstream, the O2 sensor reacts to the gases as it exits the converter, while the upstream O2 sensor in the exhaust reacts to the gases as they exit the car’s engine.

Common Symptoms

The most common symptom of this failure to the driver is the presence of the malfunction indicator lamp or the MIL. It is unlikely that the driver will notice any problems while they are driving, although there could be symptoms to this. For example, if the substance inside the catalytic converter is broken. It could restrict the exhaust which can cause reduction of power output from the vehicle.

Possible Causes

The P0420 code error may mean that one or more of the following has happened:

  • Leaded fuel was used where unleaded was called for
  • A damaged or failed oxygen sensor (HO2S)
  • Downstream oxygen sensor (HO2S) wiring damaged or connected improperly
  • The engine coolant temperature sensor is not working properly
  • Damaged or leaking exhaust manifold / catalytic converter / muffler / exhaust pipe Retarded spark timing
  • The oxygen sensors in front and behind the converter are reporting too similar of readings
  • Leaking fuel injector or high fuel pressure
  • Cylinder misfire
  • Oil contamination

How to Check

To check and see if the catalytic converter has actually failed is not difficult, and it’s a process which can easily be performed at home.

There are 4 simple tests that you can do to see if the catalytic converter is in bad shape. All you have to do is:

  1. Check that the catalytic converter’s filter element hasn’t shattered.

All you have to do in checking if the catalytic converter’s filter element has shattered is to bang on it with the palm of your hand. If it’s busted inside, you’ll hear the pieces rattling.

It is important to note that the test has to be done with a completely cold engine!

  1. Verifying that the downstream oxygen sensor is showing a rich condition.

This has to be done with a scan tool in Live Data mode.

  1. Induce a lean condition to see if the downstream oxygen sensor reports it.

This is done by removing a large vacuum hose and checking to see if the downstream oxygen sensor reports the vacuum leak. You will need to use your scan tool in Live Data mode.

  1. Introduce a rich condition to see if the downstream oxygen sensor reports it.

This is done by spraying a small amount of carburetor cleaner into a small vacuum hose, usually the fuel pressure regulator’s vacuum hose/line, and checking to see if the rear O2 sensor reports the addition of the spray. You will also need to use your scan tool in Live Data mode.

How to Fix

There are several recommended steps for fixing a P0420 error code, this includes:

  • You can check for exhaust leaks at the manifold, pipes, and catalytic converter. Repair as required.
  • You can use a scope to diagnose the oxygen sensor operation (Tip: The oxygen sensor in front of the catalytic converter normally has a fluctuating waveform. The waveform of the sensor behind the converter should be more steady).
  • You can inspect the downstream heated oxygen sensor (HO2S), replace if necessary.
  • You can replace the catalytic converter

Overall, the greatest mistake vehicles owners make when they have a P0420 code is to simply replace an oxygen sensor (H02S). It is important to consider the proper diagnosis so you’re not wasting money replacing parts unnecessarily. We recommend that if you need to replace the catalytic converter that you replace it with an OEM unit. Second choice would be a high-quality replacement part. There are many stories in our forums where folks replaced the cat with a cheaper aftermarket one only to have the code return shortly thereafter. One thing to note is that many vehicle manufacturers offer a longer warranty on emissions-related parts. So if you have a newer car but it’s out of it’s bumper-to-bumper warranty, there still may be warranty on this type of problem. Many manufacturers give a five year, unlimited mileage warranty on these items. It’s worth checking into.

For more information about different OBD II error codes, feel free to browse through our website. For professional mechanic assistance, Contact Us.