Catalyst Monitor
OBD System Description - Catalyst MonitorSystem Description / Monitoring Procedure
Exhaust oxygen concentration at the upper part and the lower part of WU-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. While the above controls is going on and if WU-TWC is in good condition, the output voltage of HO2S-2 is maintained at specified level. As WU-TWC becomes deteriorated, even when the above control is going on, the exhaust gas which has passed WU-TWC then passes HO2S-2 at the exhaust oxygen concentration similar to that of the upper part of WU-TWC without being oxygenated or converted. Thus, waveforms of HO2S-1 and HO2S-2 output voltage become alike, ECM (PCM) judges deterioration of WU-TWC by comparing waveforms of HO2S-1 and HO2S-2.
Catalyst System Layout
The system has 2 oxygen sensors located upstream and downstream of the catalysts. The layout of catalysts and oxygen sensors is described below.
DTC P0420: Catalyst System Efficiency Below Threshold
Wiring Diagram:
System / Wiring Diagram
Circuit Description
ECM 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 warm up three way catalytic converter and three way catalytic converter.
Reference
Oscilloscope waveforms
DTC Detecting Condition:
DTC Detecting Condition and Trouble Area
DTC Confirmation Procedure
WARNING:
- When performing a road test, select a place where there is no traffic or possibility of a traffic accident and be very careful during testing to avoid occurrence of an accident.
- Road test should be carried out with 2 persons, a driver and a tester, on a level road.
NOTE: Check to make sure that the following conditions are satisfied when using this DTC Confirmation Procedure.
- Intake air temp.: Between 14 °C and 70 °C (6.8 °F and 158 °F)
- Altitude (barometric pressure): 2500 m, 8202 ft or less (560 mmHg, 75 kPa or more)
1. Connect scan tool to DLC with ignition switch turned OFF.
2. Turn ON ignition switch and clear DTC using scan tool.
3. Increase vehicle speed to 50 - 60 mph, 80 - 100 km/h. (engine speed: 2500 - 3000 r/min.)
4. Keep the vehicle speed for 10 min. or more (Throttle valve opening is kept constant in this step).
5. Stop vehicle and check if DTC / pending DTC exists using scan tool. If not, check if catalyst monitoring test has completed using scan tool. If not in both of the checks (i.e., no DTC / pending DTC and catalyst monitoring test not completed), check vehicle condition (environmental) and repeat step 3 through 5.
Step 1 - 3:
DTC Troubleshooting
Operation:
Enable Conditions:
Typical Malfunction Thresholds:
Catalyst Monitor
MODE $06 Data:
MODE $06 Data