Suzuki Engine 2002 GRAND VITARA SQ625, Service Manual

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6-100 ENGINE GENERAL INFORMATION AND DIAGNOSIS (G16/J20 ENGINES)
DTC P0420 Catalyst System Efficiency Below Threshold
SYSTEM DIAGRAM
CIRCUIT DESCRIPTION
Exhaust oxygen concentration at the pre-TWC and the post-TWC is detected from HO2S-1 and HO2S-2
respectively and accordingly ECM (PCM) controls the closed loop which then controls the fuel injection volume.
(Refer to “Fuel Injector” in Section 6E1.) While the above control is going on and if TWC is in good condition, the
output voltage of HO2S-2 is maintained at specified level. As TWC becomes deteriorated, even when the above
control is going on, the exhaust gas which has passed TWC then passes HO2S-2 at the exhaust oxygen con-
centration similar to that of the pre-catalyst without being oxygenated or converted. Thus, waveforms of HO2S-1
and HO2S-2 output voltages become alike. ECM (PCM) judges deterioration of TWC by comparing waveforms
of HO2S-1 and HO2S-2.
DTC DETECTING CONDITION AND TROUBLE AREA
[A] : Canvas top model 2. Heated oxygen sensor-2 5. ECM (PCM)
[B] : Other than canvas top model 3. Warm up three way catalytic converter (WU-TWC, if equipped)
1. Heated oxygen sensor-1 4. Three way catalytic converter
DTC DETECTING CONDITION TROUBLE AREA
While running under conditions described for “DTC Confirma-
tion Procedure”, output waveform of HO2S-1 becomes simi-
lar to that of HO2S-2.
(2 driving cycles detection logic) • Exhaust gas leakage
• Three way catalytic convertor
• Heated oxygen sensor-2 or its circuit
• ECM (PCM)