P0431

DTC P0431

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 engine control module (ECM) monitors this process by using a heated oxygen sensor (HO2S) which is in the exhaust stream past the three-way catalytic converter, in each bank. The HO2S produces an output signal which indicates the oxygen storage capacity of the catalyst. This in turn indicates the catalysts ability to convert the exhaust emissions effectively. The ECM monitors the catalyst efficiency by first allowing the catalyst to heat, then waiting for a stabilization period while the engine is idling. The ECM then adds and removes fuel while monitoring the reaction of the HO2S. When the catalyst is functioning properly, the HO2S 2 response to the extra fuel is slow compared to the HO2S 1. When the HO2S 2 response is close to that of the HO2S 1, the oxygen storage capability or efficiency of the catalyst is considered to be degraded below an acceptable threshold. If the ECM detects the degraded condition DTC P0421 for bank 1 or P0431 for bank 2 sets.

CONDITIONS FOR RUNNING THE DTC
- DTCs P0031, P0037, P0038, P0050, P0051, P0052, P0056, P0057, P0058, P0101, P0102, P0103, P0107, P0108, 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 MAP sensor variation is less than 6 kPa.
- DTCs P0421 and P0431 run once for 50 seconds when the above conditions are met. If the test fails, the ECM will perform one more test during the ignition cycle for the same amount of time.

CONDITIONS FOR SETTING THE DTC
The ECM has determined the catalyst efficiency has degraded below a calibrated threshold.

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.