Suzuki Wagon R Manual page 1166

6-1-74 GENERAL INFORMATION AND ENGINE DIAGNOSIS
DTC P0420 CATALYST SYSTEM EFFICIENCY BELOW THRESHOLD
CIRCUIT DESCRIPTION
Heated
oxygen
sensor-1
Warm up three way
catalytic converter
ECM (PCM) monitors oxygen concentration in the exhaust gas which has passed the three way catalytic converter
by HO2S-2.
When the catalyst is functioning properly, the variation cycle of HO2S-2 output voltage (oxygen concentration) is
slower than that of HO2S-1 output voltage because of the amount of oxygen in the exhaust gas which has been
stored in the catalyst.
Reference
Oscilloscope Waveforms
DTC DETECTING CONDITION
D While vehicle running at constant speed under other
than high load.
D Time from rich or lean switching command is output
till HO2S-2 output voltage crosses 0.45 V is less than
specified value.
2 driving cycle detection logic, monitoring once/1
driving.
To other sensors
Three way catalytic converter
0.5 V / Div
2 sec. / Div
Engine running at
Closed loop condition
Heated oxygen sensor-2
Fuel
Cut
POSSIBLE CAUSE
D Exhaust gas leak
D Three way catalytic converter malfunction
D Fuel system malfunction
D HO2S-2 malfunction
D HO2S-1 malfunction
Idle after fuel cut