P0420
DTC P0420
Diagnostic Instructions
* Perform the Diagnostic System Check - Vehicle (Initial Inspection and Diagnostic Overview) prior to using this diagnostic procedure.
* Review Strategy Based Diagnosis (Initial Inspection and Diagnostic Overview) for an overview of the diagnostic approach.
* Diagnostic Procedure Instructions (Initial Inspection and Diagnostic Overview)provides an overview of each diagnostic category.
DTC Descriptor
DTC P0420
- Catalyst System Low Efficiency
Circuit/System Description
A 3-way catalytic converter (TWC) controls 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 are present in the exhaust gas. This process converts the HC and the CO into water vapor and carbon dioxide (CO2), and reduces 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) that is in the exhaust stream after the TWC. The HO2S 2 produces an output signal that the ECM uses to calculate the oxygen storage capacity of the catalyst. This indicates the ability of the catalyst to convert the exhaust emissions efficiently. The ECM monitors the efficiency of the catalyst by allowing the catalyst to heat, then wait for a stabilization period while the engine is idling. The ECM then adds and removes fuel while monitoring the HO2S 2. When the catalyst is functioning properly, the HO2S 2 response to the extra fuel is slow compared to the response of the HO2S 1, which is located before the TWC. When the HO2S 2 response is near that of the HO2S 1, the oxygen storage capability and efficiency of the catalyst may be degraded below an acceptable threshold.
Conditions for Running the DTC
* DTCs P0016, P0031, P0032, P0037, P0038, P0106, P0107, P0108, P0110, P0111, P0112, P0113, P0115, P0116, P0117, P0118, P0122, P0123, P0125, P0128, P0131, P0132, P0133, P0134, P0135, P0137, P0138, P0140, P0141, P0171, P0172, P0201, P0202, P0203, P0204, P0217, P0261, P0262, P0264, P0265, P0267, P0268, P0270, P0271, P0300, P0335, P0336, P0337, P0340, P0351, P0352, P0402, P0404, P0405, P0406, P0441, P0443, P0444, P0445, P0488, P0496, P0502, P0506, P0507, P0562, P0563, P1137, P1138, P1166, P2110, P2187, P2188, P2195, P2196, and P2297 are not set.
* Before the ECM performs the idle test, the vehicle must be driven under the following conditions:
- The calculated air flow into the engine is more than 9 g/s for more than 16 seconds for a manual transmission.
- The calculated air flow into the engine is more than 11 g/s for more than 13 seconds for an automatic transmission.
* The engine has been running for more than 9.5 minutes.
* The throttle angle is less than 1.5%.
* The EVAP purge concentration is learned.
* The long term fuel trim is learned.
* The air flow into the engine is between 2-8 g/s.
* The engine has been idling less than 60 seconds.
* The vehicle speed is less than 3 km/h (2 mph).
* The engine coolant temperature (ECT) is between 70°-109°C (160°-228°F).
* The intake air temperature (IAT) is between -7°C and +105°C (+19° and +221°F).
* The barometric pressure (BARO) is more than 72 kPa.
* The catalytic converter (TWC) calculated temperature is between 300°-920°C (662°-1,688°F).
* The vehicle is running in Closed Loop.
* Torque fuel reduction is not active.
* The transmission is not in P/N (automatic transmission only).
* The engine speed is steady within 80 RPM.
* The A/C clutch is not changing state.
* The cooling fan is not changing state.
This diagnostic attempts one test during each valid idle period once the above conditions have been met. This diagnostic attempts up to 12 tests during each drive cycle.
Conditions for Setting the DTC
The ECM determines that the efficiency of the catalyst has degraded below a calibrated threshold.
Action Taken When the DTC Sets
DTC P0420 is a Type A DTC.
Conditions for Clearing the DTC
DTC P0420 is a Type A DTC.
Diagnostic Aids
Inspect for the following conditions, which may cause a catalytic converter to degrade:
* 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 Testing for Intermittent Conditions and Poor Connections (Component Tests and General Diagnostics).
Reference Information
Schematic Reference
Engine Controls Schematics (MR140 w/o Throttle Actuator Control) (Electrical Diagrams)Engine Controls Schematics (MT34 w/Throttle Actuator Control) (Electrical Diagrams)Engine Controls Schematics (Hatchback) (Electrical Diagrams)
Electrical Information Reference
* Circuit Testing (Component Tests and General Diagnostics)
* Connector Repairs (Component Tests and General Diagnostics)
* Testing for Intermittent Conditions and Poor Connections (Component Tests and General Diagnostics)
* Wiring Repairs (Component Tests and General Diagnostics)
DTC Type Reference
Powertrain Diagnostic Trouble Code (DTC) Type Definitions (Diagnostic Trouble Code Descriptions)
Scan Tool Reference
Control Module References (Programming and Relearning)for scan tool information
Circuit/System Testing
1. Engine runnng, observe the DTC information with a scan tool. Verify there are no HO2S or misfire DTCs set.
• If any of the DTCs are set, refer to Diagnostic System Check - Vehicle (Initial Inspection and Diagnostic Overview) to diagnosis the applicable DTC, before proceeding with this diagnostic procedure.
2. Verify the following conditions do not exist with the catalytic converter:
* Dents
* A severe discoloration caused by excessive temperatures
* Road damage
* An internal rattle caused by damaged catalyst substrate
* Restrictions
• If a condition is found, replace the catalytic converter.
3. Verify the following conditions do not exist with the exhaust system:
* Leaks
* Physical damage
* Loose or missing hardware
* Properly torqued HO2S
• If a condition is found, repair the exhaust system.
4. Verify the following conditions do not exist with the HO2S 2:
* A grounded wiring harness
* Damage
• If a condition is found, replace the HO2S 2.
5. If no physical condition is detected and the HO2S 2 is as active as the HO2S 1 after operating the engine at 1,500 RPM for 1 minute and then returning to a stabilized idle, then replace the catalytic converter.
Repair Instructions
Perform the Diagnostic Repair Verification (Verification Tests) after completing the diagnostic procedure.
* Heated Oxygen Sensor 2 Replacement (Service and Repair)
* Catalytic Converter Replacement (Service and Repair)
* Exhaust Leakage (Exhaust Leakage)
* Restricted Exhaust (Restricted Exhaust)