P0420

DTC P0420

CIRCUIT DESCRIPTION
The three-way catalytic converter (TWC) reduces 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 CO that are present in the exhaust gas. This process converts these chemicals into water vapor and carbon dioxide (CO2), and will reduce the NOx by converting them into nitrogen. The catalytic converter also stores oxygen. The powertrain control module (PCM) monitors this process using heated oxygen sensor (HO2S) bank 1 sensor 2 and HO2S bank 2 sensor 2, located in the exhaust stream after the TWC. These sensors are referred to as the catalyst monitor sensors. The catalyst monitor sensors produce an output signal that the PCM uses to indicate the oxygen storage capacity of the catalyst. This determines the catalyst's ability to effectively convert the exhaust emissions.

If the catalyst is functioning correctly, the HO2S bank 1 sensor 2 and HO2S bank 2 sensor 2 signals will be far less active than the signals that are produced by HO2S bank 1 sensor 1 and HO2S bank 2 sensor 1. This indicates that the TWC oxygen storage capacity is at an acceptable threshold. When the response time of the catalyst monitor sensors are close to that of the fuel control sensors, the ability of the catalyst to store oxygen may be below an acceptable threshold.

The PCM performs this diagnostic test at idle. When the Conditions for Running this DTC are met, the following occurs:
- The air-to-fuel ratio transitions from lean to rich.
- The air-to-fuel ratio transitions a second time rich to lean, opposite the first air-to-fuel ratio transition.
- The PCM captures the response time of the front and the rear HO2S when the air-to-fuel ratio transitions occur. The HO2S response time changes from less than 350 mV to more than 600 mV, and from more than 600 mV to less than 350 mV.
- The PCM measures the time necessary for the rear HO2S voltage to cross a reference lean-to-rich threshold, and the time necessary for the front HO2S voltage to cross the same lean-to-rich threshold. The difference between the front HO2S time and the rear HO2S time indicates the oxygen storage capacity of the catalyst. If the PCM detects that this time difference is less than a predetermined value, DTC P0420 for bank 1.

DTC DESCRIPTOR
This diagnostic procedure supports the following DTC:
DTC P0420 Catalyst System Low Efficiency Bank 1

CONDITIONS FOR RUNNING THE DTC
- DTCs P0016, P0053, P0054, P0059, P0101, P0102, P0103, P0106, P0107, P0108, P0112, P0113, P0116, P0117, P0118, P0121, P0122, P0123, P0128, P0131, P0132, P0133, P0134, P0135, P0137, P0138, P0140, P0141, P0151, P0152, P0153, P0154, P0155, P0171, P0172, P0174, P0175, P0200, P0300, P0335, P0336, P0341, P0351, P0442, P0443, P0446, P0452, P0453, P0455, P0496, P0500, P0502, P0503, P0506, P0507, P1133, P1134, P1153, P1154, P2A01 are not set.
- The intake air temperature (IAT) is between -28 and +85°C (-20 and +185°F).
- The barometric pressure (BARO) is more than 70 kPa (10.2 psi).
- The engine coolant temperature (ECT) is between 70-125.5°C (158-258°F).
- Since the end of the last idle period, the engine speed has been more than 750 RPM for 39 seconds.
- The Closed Loop fuel control is enabled.
- The engine must be at a stable idle speed within 200 RPM of desired idle.
- This diagnostic attempts up to one test during each valid idle period once the above conditions have been met for 5 seconds.
- This diagnostic attempts up to 12 tests during each 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 as few as one test attempt, however, this diagnostic may require as many as 18 test attempts, which would require 3 ignition cycles. Each test attempt occurs within 1.5 minutes.

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.

DIAGNOSTIC AIDS
- The catalyst test may abort due to a change in the engine load. DO NOT change the engine load. Ensure that the A/C is OFF and the engine cooling fans are not cycling while a catalyst test is in progress.
- 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 the condition is determined to be intermittent, refer to Intermittent Conditions. Intermittent Conditions
- 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.

TEST DESCRIPTION

Steps 1 - 7:

Steps 8 - 11:

The numbers below refer to the step numbers on the diagnostic table.
5. This step indicates that a catalytic converter which has been discolored may be due to an engine running rich, lean, or had a previous severe misfire. Verifying the fuel trim percentages may be of assistance in determining if such a condition exists.
6. This step inspects for conditions that can cause the TWC efficiency to appear degraded.