General Information

P0420 Catalyst System Efficiency below Threshold (Bank 1)

General Information

Component Location





General Description

The catalyst's efficiency is demonstrated by its ability to oxidize CO and hydrocarbon emissions. The Powertrain Control Module (ECM) compares the output signals of the front and rear oxygen sensors to determine whether the output of the rear sensor is beginning to match the output of the front oxygen sensor. Air/fuel mixture compensation keeps the frequency of the front oxygen sensor high due to the changes from rich-to-lean combustion. The catalyst causes the rear oxygen sensor to have a lower frequency. As the catalyst wears, the rear oxygen sensor's signal trace begins to match the front oxygen sensor's signal trace. That is because the catalyst becomes saturated with oxygen and cannot use the oxygen to convert hydrocarbon and CO into H2O and CO2 with the same efficiency as when it was new. A completely worn catalyst shows a 100% match between the frequency of the front and rear sensors.

DTC Description

If amplitude of S2 is different from the model value during 60 sec under enable conditions, ECM sets DTC P0420.

DTC Detecting Condition





** S1 : upstream oxygen sensor / S2 : downstream oxygen sensor

Signal Waveform and Data




If you release the accelerator pedal suddenly after engine running about 4000 rpm, fuel supply will stop for short period and the O2 sensor service data in the GDS (Pro) will display values 200 mV or lower. When you suddenly press on the accelerator pedal down, the voltage will reach 0.6 - 1.0 V. When you let the engine idle again, the voltage will fluctuate between 200 mV or lower and 0.6 - 1.0 V. In this case, the O2sensor can be determined as good.