Toyota DTC P0420

Catalyst System Efficiency Below Threshold – Bank 1

What it means: OBD-II code P0420 represents a problem with the catalyst system on your Toyota.

The catalytic converter on your Toyota is an integral part of your vehicle’s exhaust system. This part looks like a small muffler, but its purpose is to reduce vehicle emissions through the exhaust.

Toyota DTC P0420

In front of and behind the the catalytic converter are oxygen sensors. When the Toyota is running in a closed loop, fluctuation should occur in the upstream oxygen sensor waveform. The stream moving down should be level. Most of the time, if the code P0420 has been triggered, the ready of each sensor is similar.

What this ultimately indicates is that the catalytic converter is not functioning as it should. While catalytic converters don’t typically fail out of the blue, the issue is more than likely symptomatic of another issue that caused it to fail.

When your engine control module, or ECM, has stored the code P0420, it is likely due to one of these reasons:

  • Leaded fuel was used in a vehicle requiring unleaded gasoline
  • An oxygen sensor is damaged or has failed
  • Downstream O2 sensor wiring is damaged or loose
  • Engine coolant sensor is not working
  • Damage or leaks are present in the exhaust manifold, catalytic converter, muffler, or exhaust pipes
  • Spark timing is too far retarded
  • A fuel injector is leaking
  • Cylinder misfire
  • Oil contamination

Troubleshooting should begin with a visual inspection of all involved components. Check for exhaust leaks, damaged wiring, or visual indications of a bad sensor. An inspect and replacement of downstream O2 sensors is needed, as it’s the most common cause that triggers this code. In some circumstances, the catalytic converter may need to be replaced.

If you’re getting code P0420, we suggest consulting with a professional technician before making any major repairs or replacements. This guide is meant to explain the most common causes that trigger the diagnostic trouble code P0420, and it is not meant to replace professional troubleshooting services.