Suzuki 2000 Grand Vitara SQ625 Service Manual page 88

6-84 ENGINE 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 respec-
tively and accordingly ECM (PCM) controls the closed loop which then controls the fuel injection volume. (Refer
to 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 ex-
haust gas which has passed TWC then passes HO2S-2 at the exhaust oxygen concentration 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
DTC DETECTING CONDITION
While running under conditions described for DTC
CONFIRMATION PROCEDURE, output waveform of
HO2S-1 becomes similar to that of HO2S-2.
(2 driving cycles detection logic)
1. Heated oxygen sensor-1
2. Heated oxygen sensor-2
3. Warm up three way catalytic converter
(WU-TWC, if equipped)
4. Three way catalytic converter
TROUBLE AREA
D Exhaust gas leakage
D Three way catalytic convertor
D Heated oxygen sensor –2 or its circuit
D ECM (PCM)