P0420

System Description
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 NOx, by converting NOx into nitrogen. The PCM has the capability to monitor this process using a Heated Oxygen Sensor (HO2S). The HO2S is located in the exhaust stream past the three - way catalytic converter. The HO2S 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. Whenever the catalyst is functioning correctly, the HO2S signal will be far less active than the signal produced by the front Oxygen Sensor (O2S). The PCM monitors the catalyst efficiency in two stages. When the catalyst efficiency falls below 75%, the PCM will set a stage 1 fail flag but will not illuminate the Malfunction Indicator Lamp (MIL) at this point. When catalyst efficiency falls below 50%, the PCM will set a stage 2 fail flag and the MIL will be illuminated.

Conditions for Setting the DTC
- The HO2S 2 voltage varies greater than 250 mV
- The engine speed is between 1700 and 3700 RPM
- The calculated Air Flow is between 12 and 29 GPS
- The vehicle speed is between 30 and 80 MPH
- The Engine Coolant Temperature (ECT) is greater than 70°C (158°F)
- One or more of the following DTCs are not set P0106, P0107, P0108, P0112, P0113, P0117, P0118, P0121, P0122, P0123, P0131, P0132, P0133, P0134, P0137, P0138, P0140, P0141, P0171, P0172, P0300, P0301, P0302, P0303, P0304, P0502, P0506, P0507, P0601, P0503, P0562, P0563, P1133, P1406, P1441.

Action Taken When the DTC Sets
- The MIL will illuminate.
- The PCM 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 three 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.
- DTCs can be cleared by using a scan tool.

Diagnostic Aids
- 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 backed out terminals, improper mating, broken locks, or physical damage to the wiring harness.
- Refer to !intermittents, in "Diagnosis by Symptom".

DTC P0420 - Catalyst System Efficiency Below Threshold:

Test Description
Numbers below refer to the step numbers on the Diagnostic Table.
1. The OBD System Check prompts the Technician to complete some basic checks and store the Freeze Frame Data and the Failure records on the scan tool if applicable. This creates an electronic copy of the data taken when the fault occurred. This information on the scan tool can be referred to later.
2. Whenever component DTCs are set, diagnose those DTCs first. A fault in a component may cause the three way catalytic converter to appear degraded or may have caused the converters fault.
3. This checks for conditions that can cause the three-way catalytic converter to appear degraded. Repair any problems found before proceeding with this Table.
5. Whenever the three-way catalytic converter needs to be replaced, make sure that another condition is not present which would caused the converter to become damaged. These conditions may include: misfire; high engine oil or coolant consumption; retarded spark timing or weak spark. To avoid damaging the replacement converter, correct any possible causes of converter damage before replacing the catalytic converter.