Brand:
Toyota
Model Name:
Yaris NCP90 91 92 93
Manual:
Repair Manual
Section:
1Nz-Fe Engine Control System
Title:
Sfi System
DTC P0420 Catalyst System Efficiency Below Threshold (Bank 1)
DESCRIPTION
The ECM uses the heated oxygen sensors mounted in front of and behind the Three-Way Catalytic Converter (TWC) to monitor its efficiency. The front heated oxygen sensor (sensor 1) sends pre-TWC air-fuel ratio information to the ECM, and the rear heated oxygen sensor (sensor 2) sends post-TWC air-fuel ratio information to the ECM. The ECM compares the 2 signals to assess the efficiency of the TWC and the TWC's ability to store oxygen. During normal operation, the TWC stores and releases oxygen as needed. This capacity to store oxygen reduces the variation in the post-TWC exhaust stream as shown below.If the TWC is functioning normally, the waveform of sensor 2 slowly fluctuates between RICH and LEAN.
As the TWC efficiency deteriorates, its ability to store oxygen reduces and the sensor output becomes more variable.
When the TWC monitor runs, the ECM measures the signal lengths of both sensors 1 and 2, and then calculates the ratio of the signal lengths (TWC deterioration level). If the TWC deterioration level exceeds the threshold, the ECM interprets this as a TWC malfunction. The ECM illuminates the MIL and sets the DTC.
| DTC No. | DTC Detection Conditions | Trouble Areas |
| P0420 | The ratio of the signal lengths (the signal length of oxygen sensor 2 / the signal length of oxygen sensor 1) smaller than standard value (2 trip detection logic) |
|
CONFIRMATION DRIVING PATTERN
(a) Connect an intelligent tester to the DLC3.
(b) Start the engine and warm it up with all the accessories switched off until the engine coolant temperature becomes stable.
(c) Run the engine at 2,500 to 3,000 rpm for about 3 minutes.
(d) Confirm that the heated oxygen sensor (sensor 1) has the waveform that is around 0.5 V during feedback to the ECM. Then check the waveform of the heated oxygen sensor (sensor 2) for the same waveform (0.5 V during feedback).
- HINT:
- If there is a malfunction in the TWC, the waveform of the heated oxygen sensor (sensor 2) is almost the same as that of the heated oxygen sensor (sensor 1).
WIRING DIAGRAM
INSPECTION PROCEDURE
- HINT:
- Read freeze frame data using an intelligent tester. Freeze frame data records the engine conditions when a malfunction is detected. When troubleshooting, freeze frame data can help determine if the vehicle was running or stopped, if the engine was warmed up or not, if the air-fuel ratio was LEAN or RICH, and other data from the time the malfunction occurred.
1.CHECK ANY OTHER DTCS OUTPUT (IN ADDITION TO DTC P0420)
Connect an intelligent tester to the DLC3.
Turn the ignition switch to ON and turn the tester on.
Select the following menu items: Powertrain / Engine and ECT / DTC.
Read DTCs.
- Result:
Display (DTC Output) Proceed To P0420 A P0420 and other DTCs B
- HINT:
- If any DTCs other than P0420 are output, troubleshoot those DTCs first.
| GO TO DTC CHART (Click here) | |||
| A | |
 |
2.PERFORM ACTIVE TEST USING INTELLIGENT TESTER (CONTROL THE INJECTION VOLUME)
Connect an intelligent tester to the DLC3.
Turn the ignition switch to ON.
Warm up the engine by running the engine at 2,500 rpm for approximately 90 seconds.
Select the following menu items: Powertrain / Engine and ECT / Active Test / Control the Injection Volume for A/F Sensor.
Perform the Active Test while the engine is idling.
- Standard:
- Heated oxygen sensor reacts in accordance with the increases and decreases in injection volume:
+25% → RICH output: More than 0.5 V
-12.5% → LEAN output: Less than 0.4 V
- NOTICE:
- The heated oxygen sensor (sensor 1) has an output delay of few seconds and the heated oxygen sensor (sensor 2) has a maximum output delay of 20 seconds.
| Case | Heated Oxygen Sensor (Sensor 1) Output Voltage | Heated Oxygen Sensor (Sensor 2) Output Voltage | Main Suspected Trouble Area |
| 1 |   |  | - |
| 2 |  | |
|
| 3 | | |
|
| 4 | | |
|
| Result | Proceed To |
| Case 1 | A |
| Case 2 | B |
| Case 3 | C |
| Case 4 | D |
| REPLACE HEATED OXYGEN SENSOR (SENSOR 1) (Click here) | |||
| Go to step 4 | |||
| CHECK AND REPAIR CAUSES OF EXTREMELY RICH OR LEAN ACTUAL AIR-FUEL RATIO, AND GO TO STEP 3 | |||
| A | |
|
3.CHECK FOR EXHAUST GAS LEAK
- OK:
- No gas leakage.
| REPAIR OR REPLACE EXHAUST GAS LEAKAGE POINT | |||
| OK | |
 | |
| REPLACE THREE-WAY CATALYTIC CONVERTER (BOTH FRONT AND REAR CATALYSTS (FRONT EXHAUST PIPE)) | |
4.CHECK FOR EXHAUST GAS LEAK
- OK:
- No gas leakage
| REPAIR OR REPLACE EXHAUST GAS LEAKAGE POINT | |||
| OK | |
| |
| REPLACE HEATED OXYGEN SENSOR (SENSOR 2) (Click here) | |