P0420
Diagnostic Chart:
Circuit Description
In order to control emissions of Hydrocarbons (HC), Carbon Monoxide (CO), and Oxides of Nitrogen (NOx), a Three-Way Catalytic (TWC) converter 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. The catalyst also reduces the emission of NOx by converting it to nitrogen. The PCM has the ability to monitor this process using the HO2S 1 and the HO2S 2 heated oxygen sensors. The HO2S 1 sensor produces an output signal which indicates the amount of oxygen present in the exhaust gas entering the three-way catalytic converter. The HO2S 2 sensor produces an output signal which indicates the oxygen storage capacity of the catalyst; this in turn indicates the catalysts ability to convert exhaust gases efficiently. If the catalyst is operating efficiently, the HO2S 1 signal will be far more active than that produced by the HO2S 2 sensor. If the PCM detects a level of HO2S 2 activity that indicates the catalyst is no longer operating efficiently, a DTC P0420 will be set.
Conditions for Setting the DTC
Calculated deviation rate:
- Engine coolant temperature between 70°C (158°F) and 110°C (230°F).
- Intake air temperature between -10°C (14°F) and 70°C (158°F).
- Barometric pressure greater than 75 kPa.
- Fuel tank level greater than 15%.
- Engine Speed between:
- M/T2750 and 3750 RPM.
- AFT 1500 and 3500 RPM.
- Calculated load value between 15 and 50%.
- In closed loop mode 139 seconds at 3000 RPM from engine start.
- Above conditions present for 96 seconds.
Action Taken When the DTC Sets
- The Malfunction Indicator Lamp (MIL) will illuminate after two consecutive ignition cycles in which the diagnostic runs with the fault active.
- The PCM will record operating conditions at the time the diagnostic fails. This information will be stored in the Freeze Frame buffer.
Conditions for Clearing the MIL/DTC
- The MIL will turn off after three consecutive drive cycles without a fault present.
- A DTC will clear after 40 consecutive warm-up cycles without a fault.
- The DTCs can be cleared by using a scan tool or by disconnecting the PCM battery feed.
Diagnostic Aids
Check for any of the following conditions:
- Exhaust gas leaks.
- A three way catalytic converter malfunction.
- A fuel system malfunction.
- HO2S sensor 1 and sensor 2 malfunctions. If any HO2S DTCs are set, refer to the applicable HO2S DTC first.
- A poor connection at the PCM. Inspect the harness connectors for backed out terminal, improper mating, broken locks, improperly formed or damaged terminals, and a poor terminal to wire connection.
- A damaged wiring harness. Inspect the wiring harness for damage and repair as necessary.
If the DTC P0420 cannot be duplicated, the information included in the Freeze Frame data can be useful in determining vehicle operating conditions since the DTC was last set. This may assist in determining how often the DTC sets.
If any wiring repairs need to be made, Refer to Repair Procedures in Diagrams/Electrical.
Test Description
The numbers below refer to the step numbers in the Diagnostic Table.
1. The Powertrain (OBD) System Check prompts the technician to complete some basic checks and store the Freeze Frame data on the scan tool if applicable. This creates an electronic copy of the data taken when the fault occurred. The information is then stored in the scan tool for later reference.
2. Perform a visual/physical inspection of the catalytic converter.
5. If the three-way catalytic converter needs to be replaced, make sure that the condition that caused the converter to become damaged is no longer present. These conditions may include: misfire; retarded spark timing or weak spark; high engine oil or coolant consumption; excessively rich fuel mixture. In order to avoid damaging the replacement converter, correct any possible causes of converter damage before replacing the catalytic converter.
6. A relatively inactive HO2S sensor 2 indicates that the converter is minimizing the amount of oxygen passing through the converter.