P0420
DTC P0420: Catalyst Efficiency Below ThresholdTECHNICAL DESCRIPTION
- The signal from the rear heated oxygen sensor differs from the front heated oxygen sensor. That is because the catalytic converter purifies exhaust gas. When the catalytic converter has deteriorated, the signal from the front heated oxygen sensor becomes similar to the rear heated oxygen sensor.
- The ECM compares the output of the front and rear heated oxygen sensor signals.
DESCRIPTIONS OF MONITOR METHODS
- Front and rear heated oxygen sensor rich/lean switching frequency are nearly equal.
MONITOR EXECUTION
- Continuous
MONITOR EXECUTION CONDITIONS (Other monitor and Sensor)
Other Monitor (There is no temporary DTC stored in memory for the item monitored below)
- Heated oxygen sensor (front) monitor
- Heated oxygen sensor (rear) monitor
- Heated oxygen sensor heater (front) monitor
- Heated oxygen sensor heater (rear) monitor
- Misfire monitor
- Fuel system monitor
- Air/fuel ratio feedback system monitor
Sensor (The sensor below is determined to be normal)
- Volume air flow sensor
- Barometric pressure sensor
- Intake air temperature sensor
- Engine coolant temperature sensor
- Throttle position sensor
- Accelerator position sensor
DTC SET CONDITIONS
Logic Flow Chart:
Logic Flow Chart
Check Conditions
- Engine speed is lower than 3,500 r/min.
- Volume air flow sensor output frequency is between 94 and 238 Hz.
- The throttle valve is open.
- More than 3 seconds have elapsed after the above mentioned three conditions have been met.
- Intake air temperature is higher than -10 degree C (14 degrees F).
- Barometric pressure is higher than 76 kPa (22.4 in. Hg).
- Under the closed loop air/fuel ratio control.
- Vehicle speed is 1.5 km/h (1.0 mph) or more.
- Short-term fuel trim is higher than -25 percent and lower than +25 percent.
- The cumulative volume air flow sensor output frequency is higher than 10,000 Hz.
- The ECM monitors for this condition for 7 cycles of 10 seconds each during the drive cycle.
Judgment Criterion
- The frequency ratio of heated oxygen sensor (rear) and heated oxygen sensor (front) is more than 0.3.
OBD-II DRIVE CYCLE PATTERN
Refer to Diagnostic Function - OBD-II Drive Cycle - Pattern 4. OBD-II Drive Cycle Pattern Lists
TROUBLESHOOTING HINTS (The most likely causes for this code to be set are:)
- Catalytic converter deteriorated.
- Heated oxygen sensor failed.
- Exhaust leak.
- ECM failed.
DIAGNOSIS
Required Special Tools:
- MB991958: Scan Tool (M.U.T.-III Sub Assembly)
- MB991824: V.C.I.
- MB991827: USB Cable
- MB991911: Main Harness B
STEP 1. Check for exhaust leaks.
Q: Are there any abnormalities?
YES: Repair it. Then go to Step 7.
NO: Go to Step 2.
STEP 2. Using scan tool MB991958, check data list item 59: Heated Oxygen Sensor (rear).
CAUTION: To prevent damage to scan tool MB991958, always turn the ignition switch to the "LOCK" (OFF) position before connecting or disconnecting scan tool MB991958.
1. Connect scan tool MB991958 to the data link connector.
2. Start the engine and run at idle.
3. Set scan tool MB991958 to the data reading mode for item 59, Heated Oxygen Sensor (rear).
a. Transaxle: 2nd speed
b. Drive with wide open throttle
c. Engine: 3,500 r/min or more
- The output voltage should be between 0.6 and 1.0 volt.
4. Turn the ignition switch to the "LOCK" (OFF) position.
Q: Is the sensor operating properly?
YES: Go to Step 3.
NO: Refer to, DTC P0136 - Heated Oxygen Sensor Circuit (sensor 2), DTC P0137 - Heated Oxygen Sensor Circuit Low Voltage (sensor 2) , DTC P0138 - Heated Oxygen Sensor Circuit High Voltage (sensor 2), DTC P0139 - Heated Oxygen Sensor Circuit Slow Response (sensor 2) , DTC P0140 - Heated Oxygen Sensor Circuit No Activity Detected (sensor 2).
STEP 3. Using scan tool MB991958, check data list item 11: Heated Oxygen Sensor (front).
1. Start the engine and run at idle.
2. Set scan tool MB991958 to the data reading mode for item 11, Heated Oxygen Sensor (front).
- Warm up the engine. When the engine is revved, the output voltage should be 0.6 to 1.0 volt.
3. Turn the ignition switch to the "LOCK" (OFF) position.
Q: Is the sensor operating properly?
YES: Go to Step 4.
NO: Refer to, DTC P0130 - Heated Oxygen Sensor Circuit (sensor 1), DTC P0131 - Heated Oxygen Sensor Circuit Low Voltage (sensor 1), DTC P0132 - Heated Oxygen Sensor Circuit High Voltage (sensor 1), DTC P0133 - Heated Oxygen Sensor Circuit Slow Response (sensor 1), DTC P0134 - Heated Oxygen Sensor Circuit No Activity Detected (sensor 1).
STEP 4. Using scan tool MB991958, check data list item 11: Heated Oxygen Sensor (front).
1. Start the engine and run at idle.
2. Set scan tool MB991958 to the data reading mode for item 11, Heated Oxygen Sensor (front).
3. Keep the engine speed at 2,000 r/min.
- 0 - 0.4 and 0.6 - 1.0 volt should alternate 15 times or more within 10 seconds.
4. Turn the ignition switch to the "LOCK" (OFF) position.
Q: Is the sensor operating properly?
YES: Go to Step 5.
NO: Replace the heated oxygen sensor (front). Then go to Step 7.
STEP 5. Test the OBD-II drive cycle.
1. Carry out a test drive with the drive cycle pattern. Refer to, Diagnostic Function - OBD-II Drive Cycle - Pattern 4. OBD-II Drive Cycle Pattern Lists
2. Check the diagnostic trouble code (DTC).
Q: Is DTC P0420 set?
YES: Replace the catalytic converter. Then go to Step 6.
NO: The inspection is complete.
STEP 6. Test the OBD-II drive cycle.
1. Carry out a test drive with the drive cycle pattern. Refer to, Diagnostic Function - OBD-II Drive Cycle - Pattern 4. OBD-II Drive Cycle Pattern Lists
2. Check the diagnostic trouble code (DTC).
Q: Is DTC P0420 set?
YES: Replace the ECM. Then go to Step 7.
NO: The inspection is complete.
STEP 7. Test the OBD-II drive cycle.
1. Carry out a test drive with the drive cycle pattern. Refer to, Diagnostic Function - OBD-II Drive Cycle - Pattern 4. OBD-II Drive Cycle Pattern Lists
2. Check the diagnostic trouble code (DTC).
Q: Is DTC P0420 set?
YES: Retry the troubleshooting.
NO: The inspection is complete.