P0420-CATALYST EFFICIENCY (BANK 1)
For a complete wiring diagram Refer to Section 8W.
Theory of Operation
The catalyst monitor uses the signals from both the upstream and downstream O2 sensors to detect aging of the catalyst. As a catalyst ages, it loses some of its oxygen storing capacity. As a result, part of the untreated exhaust gases can break through the catalyst and causes the downstream O2 sensor to deviate from its neutral (Stoichiometric) position. By observing the downstream O2 signal, the degradation level of catalyst can be detected. In general, the higher the downstream O2 sensor state of change value, the more exhaust gas breakthrough and the lower the oxygen storage capacity of the catalytic converter.
- When Monitored:
ECT above 70° C (158° F), engine RPM between 1400 and 2300, and vehicle speed between 48 and 60 kph (30 and 60 mph).
- Set Condition:
If the final state of change index is within the calibrated fail threshold. Two trip fault. Three good trips to turn off the MIL.
| Possible Causes |
| EXHAUST SYSTEM LEAK |
| ENGINE MECHANICAL CONDITION |
| AGING OR CONTAMINATED O2 SENSOR |
| CATALYTIC CONVERTER |
Always perform the Pre-Diagnostic Troubleshooting procedure before proceeding. (Refer to 9 - ENGINE - STANDARD PROCEDURE).
Diagnostic Test
1. ACTIVE DTCNOTE: Diagnose and repair any O2 Sensor DTCs before continuing with this test.
NOTE: A new downstream O2 Sensor along with an aging upstream O2 Sensor may cause the DTC to set. Review the vehicle repair history before continuing.
NOTE: It may be necessary to test drive the vehicle within the DTC monitoring conditions in order for this DTC to set.
NOTE: Attempt to operate the vehicle under conditions similar to which the DTC was set. Refer to Freeze Frame information.
Start the engine and allow it to reach operating temperature.
WARNING: When the engine is operating, do not stand in direct line with the fan. Do not put your hands near the pulleys, belts, or fan. Do not wear loose clothing. Failure to follow these instructions can result in personal injury or death.
With a scan tool, select View DTCs.
Is the status Active for this DTC?
Yes
- Go to 2
No
- Refer to the *CHECKING FOR AN INTERMITTENT DTC Diagnostic Procedure. (Refer to 9 - ENGINE - DIAGNOSIS AND TESTING)
2. CATALYTIC CONVERTER
Thoroughly inspect the catalytic converter for any of the following conditions:
- Damage, dents, or holes
- Severe discoloration caused by overheating
- Internal damage
Were any problems found?
Yes
- Replace the Catalytic Converter. Repair the condition that may have caused the failure.
- Perform the PCM Verification Test. (Refer to 9 - ENGINE - STANDARD PROCEDURE)
No
- Go to 3
3. CHECKING THE EXHAUST SYSTEM FOR LEAKS
Perform the diagnostic procedure for *CHECKING THE EXHAUST SYSTEM FOR LEAKS.
Were any problems found?
Yes
- Repair as necessary.
- Perform the PCM Verification Test. (Refer to 9 - ENGINE - STANDARD PROCEDURE)
No
- Go to 4
4. ENGINE MECHANICAL CONDITION
NOTE: Check for signs of contamination that may have damaged the O2 Sensor(s) and Catalytic Converter, such as: contaminated fuel, unapproved silicone, evidence of oil, coolant or anti-seize compound. Repair as necessary.
Check the exhaust for excessive smoke caused by an internal engine problem.
Were any problems found?
Yes
- Repair the engine mechanical condition in accordance with the Service Information.
- Perform the PCM Verification Test. (Refer to 9 - ENGINE - STANDARD PROCEDURE)
No
- Go to 5
5. UPSTREAM O2 SENSOR
A new downstream O2 Sensor along with an aging upstream O2 Sensor may cause this DTC to set.
Review the vehicle repair history.
Has the downstream O2 Sensor been replaced without replacing the upstream O2 Sensor?
Yes
- Replace the upstream O2 Sensor as necessary.
- Perform the PCM Verification Test. (Refer to 9 - ENGINE - STANDARD PROCEDURE)
No
- Go to 6
6. CATALYTIC CONVERTER
If there are no possible causes remaining, view repair.
Repair
- Replace the Catalytic Converter.
- Perform the PCM Verification Test. (Refer to 9 - ENGINE - STANDARD PROCEDURE)