Troubleshooting
DTC P0420 CATALYST SYSTEM EFFICIENCY BELOW THRESHOLD (BANK 1)CIRCUIT DESCRIPTION
DTC Detection Condition:
The ECM compares the waveform of the heated oxygen sensor (HO2S) located in front of the catalyst with the waveform of the heated oxygen sensor located behind the catalyst to determine whether or not catalyst performance has deteriorated.
Air-fuel ratio feedback compensation keeps the waveform of the heated oxygen sensor in front of the catalyst alternating from rich to lean.
If the catalyst is functioning normally, the waveform of the heated oxygen sensor behind the catalyst switches back and forth between rich and lean much more slowly than the waveform of the heated oxygen sensor in front of the catalyst.
But when both waveforms change at a similar rate, it indicates that catalyst performance has deteriorated.
MONITOR DESCRIPTION
Monitor Strategy:
Typical Enabling Conditions:
Typical Malfunction Thresholds:
Monitor Result (Mode 06 Data):
The vehicle is equipped with two oxygen sensors (O2S). One is mounted upstream from the three-way catalytic (TWC) converter (Front Oxygen Sensor, "sensor 1"), the second is mounted downstream (Rear Oxygen Sensor "sensor 2"). The catalyst efficiency monitor compares sensor 1 and sensor 2 signals in order to calculate TWC ability to store the oxygen.
During normal operation, the TWC stores and releases oxygen as needed. This results in low oxygen variations in the post TWC exhaust stream as shown.
As the TWC's efficiency degrades, its ability to store oxygen is reduced. This causes higher variation in post TWC exhaust stream oxygen content and results in increased sensor 2 signal activity, as shown above. When the running the monitor, the ECM compares sensor 1 and sensor 2 signals over a specific time to determine TWC efficiency. The ECM begins by calculating the signal length for both sensors.
CONFIRMATION ENGINE RACING PATTERN
Confirmation Engine Racing Pattern:
a. Connect the hand-held tester to the DLC3, or connect the probe of the oscilloscope between terminals OX1, OX2 and E1 of the ECM connector.
b. Start engine and warm it up with all accessories switched OFF until water temp. is stable.
c. Race the engine at 2,500 to 3,000 rpm for about 3 minutes.
d. After confirming that the waveforms of the oxygen sensor, bank 1 sensor 1 (OX1), oscillate around 0.5 V during feedback to the ECM, check the waveform of the oxygen sensor, bank 1 sensor 2 (OX2).
HINT:
- If there is a malfunction in the system, the waveform of the oxygen sensor, bank 1 sensor 2 (OX2), is almost the same as that of the oxygen sensor, bank 1 sensor 1 (OX1).
- There are some cases where, even though a malfunction exists, the MIL may not illuminate.
INSPECTION PROCEDURE
Step 1 - 3:
HINT: Read freeze frame data using the hand-held tester or the OBD II scan tool. 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, as well as other data from the time when a malfunction occurred.
HINT:
Hand-held tester only:
- The following procedures enable a technician to identify the problem area if: 1) there is a malfunction in both the front and rear heated oxygen sensors but no malfunction in the catalyst converter, or 2) the actual air-fuel ratio is extremely rich or lean.
- Narrowing down the trouble area is possible by performing "A/F CONTROL" ACTIVE TEST (heated oxygen sensor or other trouble areas can be distinguished).
Perform the ACTIVE TEST's A/F CONTROL operation.
HINT: "A/F CONTROL" is ACTIVE TEST which changes the injection volume -12.5 % or +25 %.
1. Connect the hand-held tester to the DLC3 on the vehicle.
2. Turn the ignition switch ON.
3. Warm up the engine by running the engine speed at 2,500 rpm for approximately 90 seconds.
4. Select the item "DIAGNOSIS / ENHANCED OBD II / ACTIVE TEST / A/F CONTROL".
5. Perform "A/F CONTROL" with the engine in an idle condition (press the right or left button).
RESULT:
Heated oxygen sensor reacts in accordance with increase and decrease of injection volume
+25 % -> rich output: More than 0.5 V
-12.5 % -> lean output: Less than 0.4 V
NOTE: There is a delay of few seconds in the sensor 1 (front sensor) output, and there is about 20 seconds delay at maximum in the sensor 2 (rear sensor).
The following of A/F CONTROL procedure enables the technician to check and graph the voltage outputs of both the heated oxygen sensors.
To display the graph, enter the following menus: ACTIVE TEST / A/F CONTROL / USER DATA. Then select O2S B1S1 and O2S B1S2 by pressing YES. Finally, push ENTER and F4.
NOTE:
If the vehicle is short of fuel, the air-fuel ratio becomes LEAN and DTCs P0133 and/or P0153 will be recorded, and the MIL then comes on.
- If different DTCs related to different systems while terminal E2 as ground terminal are output simultaneously, terminal E2 may be open.
- Read freeze frame data using the hand-held tester or the OBD II scan tool. Freeze frame data records the engine conditions when a malfunction is detected. When troubleshooting, it is useful for determining whether the vehicle was running or stopped, the engine was warmed up or not, the air-fuel ratio was lean or rich, etc. at the time of the malfunction.
- A high heated oxygen sensor (sensor 1) voltage (0.5 V or more) could be caused by a rich air fuel mixture. Check for conditions that would cause the engine to have a rich air-fuel mixture.
- A low heated oxygen sensor (sensor 1) voltage (0.4 V or less) could be caused by a lean air fuel mixture. Check for conditions that would cause the engine to have a lean air-fuel mixture.