Circuit Description
In order to maintain a reasonably low emission level of hydrocarbons (HC), carbon monoxide (CO), and oxides of nitrogen (NOx) a 3-way catalytic converter (TWC) is used. The catalyst within the converter promotes a chemical reaction that oxidizes the HC and CO present in the exhaust gas. This reaction converts the gases into harmless water vapor and carbon dioxide. The catalyst also reduces the NOx , converting the NOx into nitrogen. The engine control module (ECM) monitors this process using the post catalyst heated oxygen sensor (HO2S) signal. The post-catalyst HO2S located in the exhaust stream after the TWC, produces an output signal that indicates the oxygen storage capacity of the catalyst. The oxygen storage capacity (OSC) determines the ability of the catalyst to convert the exhaust emissions effectively. If the catalyst is functioning correctly, the post-catalyst HO2S signal will be far less active than the signal produced by the pre-catalyst HO2S.
To determine OSC, the ECM commands a rich air/fuel mixture until all oxygen is removed from the catalyst. The ECM then commands a lean air/fuel mixture and monitors the rear heated oxygen sensors to calculate the oxygen storage capacity. The catalyst is operated in this mode until one of the following conditions occur:
| • | The oxygen stored in the catalyst exceeds a calibrated threshold, which is determined from the rear HO2S signal. |
| • | The rear HO2S indicates the catalyst to be completely saturated with oxygen, which is determined from the rear HO2S signal. |
If the ECM detects an excessive post-catalyst signal activity outside of an acceptable range for an extended period of time, this DTC sets. This indicates that the TWC oxygen storage capacity is below a threshold considered acceptable.
DTC Descriptor
This diagnostic procedure supports the following DTC:
DTC P0420 Catalyst System Low Efficiency
Conditions for Running the DTC
| • | Before the ECM can report DTC P0420 failed, DTCs P0030, P0031, P0032, P0036, P0037, P0038, P0101, P0102, P0103, P0121, P0122, P0123, P0130, P0131, P0132, P0133, P0135, P0137, P0138, P0140, P0141, P0221, P0222, P0223, P0335, P0336, P0338, P167A, P2096, P2097, P2195, P2196, P2231, P2232, P2237, P2243, P2251, P2270, P2271, P2297, and P2626, must run and pass. |
| • | DTCs P0010, P0011, P0013, P0014, P0020, P0021, P0023, P0024, P0030, P0031, P0032, P0036, P0037, P0038, P0101, P0102, P0103, P0117, P0118, P0121, P0122, P0123, P0125, P0130, P0131, P0132, P0133, P0135, P0137, P0138, P0140, P0141, P0221, P0222, P0223, P0300, P0443, P0458, P0459, P0496, P2088, P2089, P2090, P2091, P2092, P2093, P2094, P2095, P2096, P2097, P2100, P2101, P2107, P2119, P2122, P2123, P2127, P2128, P2138, P2176, P2177, P2178, P2187, P2188, P2195, P2196, P2232, P2237, P2243, P2251, P2270, P2271, P2297, and P2626, are not set. |
| • | The engine speed is between 1000-3520 RPM. |
| • | The engine load is between 18-55 percent. |
| • | The air flow into the engine is between 10-42 g/s and not changing more than 8 g/s. |
| • | The engine intake air temperature (IAT) at engine start-up is more than -9°C (-16°F). |
| • | The engine is operating for more than 320 seconds. |
| • | The engine is operating in Closed Loop. |
| • | The calculated TWC temperature is between 480°C (896°F) and 630°C (1,166°F) and steady. |
| • | DTC P0420 runs once per ignition cycle when the above conditions exist for approximately 17 minutes. The ECM will restart the test up to 3 times per ignition cycle, if the conditions above are interrupted during the test. |
Conditions for Setting the DTC
| • | The ECM determines that the catalyst efficiency has degraded below a calibrated threshold. |
| • | The condition exists for more than 4 seconds. |
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 4 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. |
Step | Action | Yes | No | ||||||
|---|---|---|---|---|---|---|---|---|---|
Schematic Reference: Engine Controls Schematics Connector End View Reference: Engine Control Module Connector End Views or Engine Controls Connector End Views | |||||||||
1 | Did you perform the Diagnostic System Check - Vehicle? | Go to Step 2 | |||||||
2 | Are any other DTCs set? | Go to Step 3 | |||||||
3 | Inspect for the following conditions:
Did you find and correct the condition? | Go to Step 5 | Go to Step 4 | ||||||
4 |
Important: Before replacing the 3-way catalytic converter (TWC), correct any conditions that may have damaged the converter. Replace the catalytic converter. Refer to Catalytic Converter Replacement . Did you complete the replacement? | Go to Step 5 | -- | ||||||
5 |
Did the DTC fail this ignition? | Go to Step 2 | Go to Step 6 | ||||||
6 | Observe the Capture Info with a scan tool. Are there any DTCs that have not been diagnosed? | System OK | |||||||
