DTC P0420: Catalyst System Low Efficiency
In order to control exhaust emissions of Hydrocarbons (HC), Carbon Monoxide (CO) and Nitrogen Oxide (NOx), a Three-Way Catalytic Converter (TWC) is used. The catalyst within the converter promotes a chemical reaction which oxidizes the HC and CO present in the exhaust gas, converting them into harmless water vapor and carbon dioxide, it also reduces NOx, converting it into nitrogen. The catalytic converter also has the ability to store oxygen. The Engine Control Module (ECM) has the capability to monitor this process using a Heated Oxygen Sensor 2 (HO2S2) located in the exhaust stream past the TWC. The HO2S2 produces an output signal which indicates the oxygen storage capacity of the catalyst; this in turn indicates the catalyst's ability to convert exhaust emissions effectively. The ECM monitors the catalyst efficiency by first allowing the catalyst to heat up, waiting for a stabilization period while the engine is idling, and then adding and removing fuel while monitoring the reaction of the HO2S2. When the catalyst is functioning properly, the HO2S2 response to the extra fuel is slow compared to the Heated Oxygen Sensor 1 (HO2S1). When the HO2S2 response is close to that of the HO2S1, the Oxygen storage capability or efficiency of the catalyst is considered to be bad, and the Malfunction Indicator Lamp (MIL) will illuminate.
Conditions for Setting the DTC
EWMA Oxygen storage time is greater than 2 seconds.
Engine run time is greater than 570 seconds.
Airflow is between 2.25 and 6.5 g/s.
Throttle position is less than 1.5015%.
Engine Coolant Temperature (ECT) is between 70 C (158 F) and 105 C (221 F).
Ambient temperature is between -7 C (-19.4 F) and 105 C (221 F).
Barometric Pressure (BARO) is greater than 72 kPa (10.4 psi).
Closed loop enabled.
Catalyst temperature is between 300 C (572 F) and 975 C (1,787 F).
Closed loop integrator change is less than 0.03.
Idle time is less than 75 seconds.
Vehicle speed is less than 3 km/h (1.9 mph).
Action Taken When the DTC Sets
The Malfunction Indicator Lamp (MIL) will illuminate.
The ECM will record operating conditions at the time the diagnostic fails. This information will be stored in the Freeze Frame and Failure Records buffers.
A history DTC is stored.
Conditions for Clearing the MIL/DTC
The MIL will turn OFF after four consecutive ignition cycles in which the diagnostic runs without a fault.
A history DTC will clear after 40 consecutive warm up cycles without a fault.
The DTC(s) can be cleared using the scan tool.
Disconnecting the ECM battery feed for more than 10 seconds.
The catalyst test may abort due to a change in the engine load. Do not change the engine load (i.E. A/C, coolant fan, heater motor) while a catalyst test is in progress.
An intermittent problem may be caused by a poor connection, rubbed-through wire insulation, or a wire that is broken inside the insulation.
Any circuitry, that is suspected as causing the intermittent complaint, should be thoroughly checked for the following conditions:
Poor terminal-to-wire connection
Number(s) below refer to the step number(s) on the Diagnostic Table.
The On-Board Diagnostic (OBD II) System Check prompts the technician to complete some basic checks and to store the Freeze Frame and Failure Records data on the scan tool if applicable. This creates an electronic copy of the data taken when the malfunction occurred. The information is then stored on the scan tool for later reference.
If any component DTCs are set, diagnose those DTCs first. A fault in a component can cause the converter to appear degraded or may have caused its failure.
This step includes checks for conditions that can cause the three-way catalytic converter to appear degraded. Repair any problems found before proceeding with this table.
If the three-way catalytic converter needs to be replaced, make sure that another condition is not present which would cause the converter to become damaged. These conditions may include the following: misfire, high engine oil or coolant consumption, and/or retarded spark timing or weak spark. To avoid damaging the replacement converter, correct any possible causes of converter damage before replacing the catalytic converter.
Clearing DTCs allows the catalyst test to be run up to 6 times this ignition cycle. Once the ignition is cycled, the test will run only once. Driving the vehicle heats the catalyst to a test temperature. The ECM must see predetermined amount of time at above idle before allowing the catalyst test to run at idle. Once at idle, the ECM will allow the system to stabilize and then test the catalyst in 2 stages.
If no faults have been found at this point and no additional DTCs were set, refer to Diagnostic Aids for additional checks and information.
Saturday, April 24th, 2010 AT 4:58 AM