6-76 ENGINE GENERAL INFORMATION AND DIAGNOSIS (TBI FOR G10)
Oscilloscope Waveforms
Engine running at
Closed loop condition
Fuel
Cut
Idle after fuel cut
0.5 V / Div
2 sec. / Div
Heated
oxygen
sensor-1
Warm up three way
catalytic converter (if equipped)
Three way catalytic converter
To other sensor
To other
sensors
Heated oxygen sensor-2
G
DTC P0420 CATALYST SYSTEM EFFICIENCY BELOW THRESHOLD
CIRCUIT DESCRIPTION
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
DTC DETECTING CONDITION POSSIBLE CAUSE
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.
D Exhaust gas leak
D Three way catalytic converter malfunction
D Fuel system malfunction
D HO2S-2 malfunction
D HO2S-1 malfunction
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