check to make sure that the wiring to the 02 sensors aren't damaged or loose, it also a possibility that the sensors could be bad too
According to AllData.....
This diagnostic procedure supports the following DTCs:
DTC P0420 Catalyst System Low Efficiency Bank 1
DTC P0430 Catalyst System Low Efficiency Bank 2
CONDITIONS FOR RUNNING THE DTC
DTCs P0030, P0036, P0050, P0053, P0054, P0056, P0059, P0060, P0068, P0101, P0102, P0103, P0106, P0107, P0108, P0112, P0113, P0116, P0117, P0118, P0120, P0128, P0131, P0132, P0133, P0134, P0135, P0136, P0137, P0138, P0140, P0141, P0151, P0152, P0153, P0154, P0155, P0156, P0157, P0158, P0160, P0161, P0171, P0172, P0174, P0175, P0178, P0179, P0200, P0220, P0300, P0335, P0336, P0341, P0342, P0343, P0351-P0358, P0442, P0443, P0446, P0449, P0452, P0453, P0455, P0496, P0500, P0502, P0503, P0506, P0507, P1125, P1133, P1134, P1153, P1154, P1258, P1516, P2101, P2108, P2120, P2121, P2125, P2135 are not set.
The intake air temperature (IAT) is between -7 and +85°C (+20 and +185°F) .
The barometric pressure (BARO) is greater than 74 kPa .
The engine coolant temperature (ECT) is more than 75-120°C (167-248°F) .
The Closed Loop fuel control is enabled.
The HO2S 1 is less than 100 mv , while Decel Fuel Cutoff parameter is active.
The HO2S 2 is less than 100 mv for 1.6 seconds , while Decel Fuel Cutoff parameter is active.
The TP Indicated Angle parameter increases 3-20 percent after exiting DFCO.
The HO2S 1 is more than 600 mv after exiting DFCO.
The HO2S 2 is more than 230 mv after exiting DFCO.
This diagnostic attempts up to one test after exiting each valid DFCO period when the above conditions have been met. This diagnostic attempts up to 18 tests per drive cycle.
CONDITIONS FOR SETTING THE DTC
The PCM determines that the oxygen storage capability of the TWC has degraded to less than a calibrated threshold.
This diagnostic may conclude in only one test attempt. However this diagnostic may require as many as 108 test attempts, which would require 6 ignition cycles. Each test attempt occurs within 10 seconds .
ACTION TAKEN WHEN THE DTC SETS
The control module illuminates the malfunction indicator lamp (MIL) when the diagnostic runs and fails.
The control module records the operating conditions at the time the diagnostic fails. The control module stores this information in the Freeze Frame/Failure Records.
CONDITIONS FOR CLEARING THE MIL/DTC
The control module turns OFF the malfunction indicator lamp (MIL) after 3 consecutive ignition cycles that the diagnostic runs and does not fail.
A current DTC, Last Test Failed, clears when the diagnostic runs and passes.
A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic.
Clear the MIL and the DTC with a scan tool.
The catalyst test may abort due to a change in the engine load. Do not change the engine load, ensure the AC is OFF, the coolant fan is not cycling, while a catalyst test is in progress.
Driving the vehicle under the conditions outlined in the Inspection/Maintenance (I/M) section can verify whether the fault is present.
These conditions may cause a catalytic converter to degrade. Inspect for the following conditions:
An engine misfire
High engine oil or high coolant consumption
Retarded spark timing
A weak or poor spark
A lean fuel mixture
A rich fuel mixture
A damaged oxygen sensor or wiring harness
If an intermittent condition cannot be duplicated, the information included in Freeze Frame data can be useful in determining the vehicle operating conditions when the DTC was set.
The catalyst may have been temporarily contaminated with a chemical from a fuel additive, fuel contamination, or any of the above conditions Chevy Parts
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