How to fix Toyota error code "P0420 – Catalyst system efficiency below threshold (Bank 1)" - a real repair case


This post will share a real case on fix error code P0420 on new model Toyota Corolla.

Model: New Toyota Corolla

Fault symptom: engine light is on, slip light is on.

Diagnose process:

First repair

Start GTS software to read the car, got error code: P0420 – Catalyst system efficiency below threshold (Bank 1), erase the code and start the car, fault lights are off. So we just thought it is occasional error. P0420 is a code occur in general exhaust system.


Second repair

Two weeks later, the fault light is on again. Reading the upstream oxygen sensor and downstream oxygen sensor both change within the range, and do a simulation test also normal, prove that two sensors works well.

For code P0420, there are mainly 3 causes:

Leak in exhaust system

Because the car only traveled more than 20,000 kilometers, the car is relatively new, and the exhaust system is unlikely to leak.

No leakage was found in the exhaust system after the inspection, which can be ruled out.

Failed sensor

Because it was very difficult to disassemble the catalytic converter, it was not disassembled at the time of the first maintenance.

A failed catalytic converter


Looked at the back end of the catalytic converter and checked that it was normal. There was no burnout. Check the sensor, many black contamination on the surface.


Because the car owner was in a hurry to go home, he drove away after only cleaned the oxygen sensor.


Third repair

After a few days, the car owner feedback the fault light is back again. We have checked the sensors last time and there is a small chance that a new car will leak, so we need to check catalytic converter. This car is a catalytic converter with the turbine, need more time to disassemble, then we found the front end of catalytic converter had been severely burned and blocked. Then we replaced the catalytic converter, the lights are off. After many days later, no lights back again.

P0420-Catalyst-system-efficiency-below-threshold-(Bank 1)


This fault code is reported when the catalytic converter efficiency is low. Why is this happening? One reason is the issue of product quality, the second is the gasoline quality.

As some reasons can cause P0420 code, we can first to check the cheaper part like sensors before we disassemble the catalytic converter to save time and money.


DISCLAIMER: The technology and software in the blog are only used for technical research and auto repair, and are not allowed to be used for illegal purposes, We will not be responsable for any direct and indirect damages from the use of the file created by this page.