P0420

DTC P0420 Catalyst System - Efficiency Below Threshold (Bank 1):

Catalyst System:

CIRCUIT DESCRIPTION
In order to control emissions of Hydrocarbons (HC), Carbon Monoxide (CO) and Oxides of Nitrogen (NOx), a three-way catalytic 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 NOx, converting it to nitrogen. The Vehicle Control Module-A (VCM-A) has the capability to monitor this process using Heated Oxygen Sensor (HO2S) 2. HO2S 2, located in the exhaust stream past the three-way catalytic converter, 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. If the catalyst is functioning correctly, the HO2S 2 signal will be far less active than that produced by HO2S 1. If a problem exists which causes the VCM-A to detect excessive HO2S 2 activity outside of an acceptable range for an extended period of time, the VCM-A will set DTC P0420, indicating that the three-way catalytic converter's oxygen storage capacity is below a threshold considered acceptable.

DTC PO420 WILL SET WHEN
^ No Vehicle Speed Sensor (VSS) DTCs set.
^ No Throttle Position (TP) sensor DTCs set.
^ Commanded Air/Fuel ratio is 14.7:1.
^ No HO2S sensor DTCs set.
^ No misfire DTCs set.
^ No Manifold Absolute Pressure (MAP) sensor DTCs set.
^ No fuel trim DTCs set.
^ No Engine Coolant Temperature (ECT) sensor DTCs set.
^ Engine coolant temperature is above 75°C (167°F).
^ Vehicle is in "Closed Loop" fuel control.
^ Above conditions met for a period of time to ensure a warm catalyst (at least 2 consecutive minutes but less than 3 min. 20 sec.)
^ Calc. eng. load is steady.
^ Engine speed is less than 4900 RPM.
^ Vehicle speed is steady between 20 mph and 70 mph.
^ Heated Oxygen Sensor (HO2S) 2 activity has been determined to be excessive by the VCM-A.
^ All above conditions for up to 2 additional minutes after catalyst warm-up criteria has been met.

ACTION TAKEN (VCM-A WILL DEFAULT TO)
The VCM-A will turn "ON" the Malfunction Indicator Lamp (MIL) (Service Engine Soon).

DTC PO420 WILL CLEAR WHEN
The VCM-A will turn the MIL "OFF" after three consecutive trips without a fault condition present. A history DTC will be cleared if no fault conditions have been detected for forty warm-up cycles [coolant temperature has risen 22°C (40°F) from start-up coolant temperature and engine coolant temperature exceeds 71°C (160°F) that same ignition cycle] or the Tech 1 clearing feature has been used.

DTC CHART TEST DESCRIPTION
Number(s) below refer to circled number(s) on the diagnostic chart.
1. This chart includes checks for conditions that can cause the three-way catalytic converter efficiency to appear degraded. Inspect and repair exhaust system as necessary. Also inspect the HO2S 2 pigtail and engine harness for any damage that can cause an intermittent fault in CKTs 1670 or 1671.
2. If the three-way catalytic converter needs to be replaced, make sure that another condition is not present which caused the catalyst to be damaged. These conditions include a) misfire; b) high engine oil consumption or coolant consumption; c) retarded spark timing or weak spark. To avoid damaging the replacement three-way catalytic converter, correct engine misfire or mechanical fault before replacing three-way catalytic converter.

DIAGNOSTIC AIDS
Difficulty running the On-Board Diagnostics (OBD) II status DTC P0420 test may be encountered in areas where test conditions cannot be maintained easily, especially in urban areas. To minimize the amount of driving required to complete the test, use the following procedure:
^ Catalyst can be warmed up in service bay previous to drive cycle.
^ Engine can be warmed up in bay also.
^ HO2S Activity Test: Using a Tech 1, monitor MAP, HO2S 1, and HO2S 2 displays in "Park" or "Neutral" above idle. Compare HO2S 1 and HO2S 2 activity (amplitude and frequency) to each other during a 30 second period. If HO2S 2 activity is nearly as great as HO2S 1 activity, a problem exists. If much less activity is noted, system is functioning properly.