P0421

DTC P0421

CIRCUIT DESCRIPTION
A 3-way catalytic (TWC) converter controls exhaust emissions of hydrocarbons (HC), carbon monoxide (CO), and oxides of nitrogen (NOx). The catalyst within the converter promotes a chemical reaction which oxidizes the HC and the CO that is present in the exhaust gas. This process will convert the HC and the CO into water vapor and carbon dioxide (CO2), and will reduce the NOx, converting the NOx into nitrogen. The catalytic converter also stores oxygen. The ECM monitors the efficiency of the catalyst by first allowing the catalyst to heat, then waiting for a stabilization period while the vehicle is at a slow and stable cruise. Secondly, the ECM will transition the air-fuel ratio lean then rich while monitoring the amplitude of the HO2S 2, then comparing this signal to a model. When the catalyst is functioning properly, the response from the HO2S 2 output signal will mirror the model. The number of test periods may vary depending on whether or not each valid test passes, fails or aborts. Each test period may need several seconds, and may need a cumulative time of 10 minutes of driving under normal conditions for the ECM to reach a decision. If the difference between the observed HO2S 2 response and the model exceeds a calibrated threshold the oxygen storage capability or efficiency of the catalyst may be degraded to an unacceptable emission standard.

DTC DESCRIPTOR
This diagnostic procedure supports the following DTCs:
- DTC P0421 Warm-Up Catalyst Low Efficiency Bank 1
- DTC P0431 Warm-Up Catalyst Low Efficiency Bank 2

CONDITIONS FOR RUNNING THE DTC
- DTCs P0031, P0037, P0038, P0050, P0051, P0052, P0056, P0057, P0058, P0101, P0102, P0103, P0121, P0122, P0123, P0128, P0130, P0131, P0132, P0133, P0134, P0135, P0136, P0138, P0140, P0141, P0150-P0158, P0159, P0160, P0161, P0171, P0172, P0174, P0175, P0206, P0221-P0223, P0300, P0301-P0306, P0431, P0440, P0442-P0449, P0506, P0507, are not set.
- The vehicle is in a Closed Loop operation.
- The catalytic temperature is more than 350°C (662°F).
- The engine load is between 20-45 percent.
- The engine speed is between 1,000-2,120 RPM.
- The Vehicle Speed Sensor (VSS) is between 40-86 kmh (25-55 mph)
- The mass air flow (MAF) sensor variation is less than 10 kPa.
- DTCs P0421 and P0431 run once within the above enabling conditions.
- This diagnostic attempts one test, once the above conditions have been met for 10 minutes. This diagnostic may attempt additional tests for more than one drive cycle.

CONDITIONS FOR SETTING THE DTC
- The ECM has determined the catalyst efficiency has degraded below a calibrated threshold.
- This diagnostic may conclude in as few as one test attempt. However, this diagnostic may require a cumulative time of 10 minutes, which may require multiple drive cycles. Each test attempt concludes within 1 second.

ACTION TAKEN WHEN THE DTC SETS
- The control module illuminates the malfunction indicator lamp (MIL) on the second consecutive ignition cycle that the diagnostic runs and fails.
- The control module records the operating conditions at the time the diagnostic fails. The first time the diagnostic fails, the control module stores this information in the Failure Records. If the diagnostic reports a failure on the second consecutive ignition cycle, the control module records the operating conditions at the time of the failure. The control module writes the operating conditions to the Freeze Frame and updates the 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.

DIAGNOSTIC AIDS
The catalyst test may abort due to a change in the engine load. Do not change the engine load. DO NOT touch the HVAC controls while a catalyst test is in progress.

Certain conditions may cause a catalytic converter to degrade. These conditions may include the following conditions:
- An engine misfire
- High engine oil or high coolant consumption
- Retarded spark timing
- A weak spark
- A lean fuel mixture
- A rich fuel mixture
- A damaged oxygen sensor wiring harness
- The Catalyst may have been temporarily contaminated with a chemical from an aftermarket fuel additive or fuels containing too much sulfur. The sulfur will coat the converter wash coat, causing the converter performance to degrade temporarily. Driving the vehicle at highway speeds for 10 minutes will burn off the contamination. Retest the converter afterwards.

TEST DESCRIPTION

Steps 1-7:

Steps 8-11:

The number below refers to the step number on the diagnostic table.
6. This step includes inspections for conditions that can cause the three-way catalytic converter to appear degraded. Before you proceed with this table, repair any conditions that you find.