P0420

DTC P0420 TWC System Low Efficiency Bank 1




Circuit Description
This diagnostic test is designed to measure the efficiency of the three-way catalytic converter (TWC) system. Efficiency is a measure of the catalytic convertor's ability to reduce the levels of hydrocarbon (HC), carbon monoxide (CO), and oxides of nitrogen (NOx) present in the exhaust gases. Catalytic converter efficiency is a function of the oxygen storage capacity (OSC) of the catalyst. The converter has the ability to store the excess oxygen present during lean conditions and then release this stored oxygen during rich conditions in order to improve HC conversion efficiency. The VCM calculates efficiency by sampling voltage from the O2 sensors located both upstream and downstream of the catalytic converters. Upstream O2 sensors provide the VCM with data on the exhaust oxygen levels entering the catalytic converter. Downstream O2 sensors provide the VCM with data on the exhaust oxygen levels exiting the catalytic converters.

Once the conditions for running this DTC are met, the VCM commands either a lean or rich air/fuel ratio depending on the current state of the exhaust oxygen level. The VCM issues a rich command if the exhaust is currently lean, or a lean command if the exhaust is currently rich. After completion of the first command, a second and opposite command is issued.

For example, if the VCM were to command a rich mixture, the upstream HO2S voltage would increase immediately. The rich mixture is delayed in reaching the downstream HO2S due to the conversion process occurring within the converter. The higher the efficiency, the greater the delay before the rich or lean mixture is detected by the downstream O2 sensor. As a result of the lower conversion efficiency within a damaged or poisoned catalyst, the delay in the rich or lean mixture reaching the downstream O2 sensor is significantly shorter. This DTC monitors the amount of time required for both the upstream and downstream HO2S voltages to cross a calibrated voltage threshold in response to the rich or lean command.

Conditions for Running the DTC
- No active secondary AIR DTCs
- No active CMP sensor DTCs
- No active ECT sensor DTCs
- No active EGR DTCs
- No active fuel trim DTCs
- No active IAC DTCs
- No active IAT sensor DTCs
- No active MAF sensor DTCs
- No active MAP sensor DTCs
- No active O2 sensor DTCs
- No active transmission DTCs
- No active EVAP system DTCs
- No active TP sensor DTCs
- No active VS sensor DTCs
- No active misfire DTCs
- The engine speed is 900 RPM or greater for more than 35 seconds since last idle period
- The engine has been running for more than 346 seconds and the long term fuel trim is stable
- The predicted catalyst temperature is greater than 475 °C (887 °F)
- The system is in closed loop
- The BARO is 73 kPa or greater.
- The IAT is between -7 - 117 °C (20 - 167 °F).
- The ECT is between 75 - 117 °C (167 - 243 °F).
- The engine has been idling for less than 60 seconds.
- The actual engine speed is within 100 RPM of the desired idle speed.

Conditions for Setting the DTC
The VCM determined that the oxygen storage capacity of the catalyst has degraded below a calibrated threshold.

Action Taken When the DTC Sets
The VCM illuminates the Malfunction Indicator Lamp (MIL) after one failure is detected.

Conditions for Clearing the MIL/DTC
- The VCM turns OFF the MIL after 3 consecutive drive trips when the test has Run and Passed.
- A history DTC will clear if no fault conditions have been detected for 40 warm-up cycles (coolant temperature has risen 22 °C (40 °F) from the startup coolant temperature and the engine coolant temperature is more than 70 °C (158 °F) during the same ignition cycle).
- Use the scan tool Clear Information function.

Diagnostic Aids
Fuel with a high sulfur content can cause a marginal convertor to look bad. Check fuel quality.

An intermittent may be caused by any of the following conditions:
- A poor connection
- Rubbed through wire insulation
- A broken wire inside the insulation

Thoroughly check any circuitry that is suspected of causing the intermittent complaint. Refer to Intermittents and Poor Connections Diagnosis.

If a repair is necessary, then refer to Wiring Repairs or Connector Repairs.

Steps 1 - 3:

Steps 4 - 5:

Step 6:

Test Description

NOTE: In order to avoid damaging the replacement three-way catalytic converter, correct the engine misfire or mechanical fault before replacing the three-way catalytic converter.

The numbers below refer to the step numbers in the diagnostic table.
3. This table checks for conditions that can cause the three-way catalytic converter efficiency to appear degraded. Inspect and repair exhaust system as necessary. Refer to Exhaust System Inspection.
6. Before the three-way catalytic converter is replaced, make sure that the following conditions are not present:
- Misfire
- High engine oil consumption or coolant consumption Retarded spark timing or weak spark.