DTC P0420: Catalyst SYS Low O2 Storage Capacity Indicated: Notes

A three-way catalytic converter is used to maintain reasonably low emissions of hydrocarbons (HC), carbon monoxide (CO) and oxides of nitrogen (NOx). The catalyst within converter promotes a chemical reaction which oxidizes HC and CO present in exhaust gas, thus converting them into harmless water vapor and carbon dioxide. The catalyst also reduces NOx, converting it to nitrogen and free oxygen. PCM is capable of monitoring this process using HO2S No. 2. HO2S No. 2, which is located in exhaust stream past 3-way catalytic converter, produces an output signal which indicates oxygen storage capacity of catalyst. This in turn indicates catalyst's ability to convert exhaust emissions effectively.

HO2S No. 2 signal will be far less active than that produced by HO2S No. 1 if catalyst is properly functioning. PCM will set Code P0420 if a problem exists which causes PCM to detect excessive HO2S No. 2 activity outside of an acceptable range for an extended period of time, indicating that 3-way catalytic converter's oxygen storage capacity is below a threshold considered acceptable. Code P0420 will set when the following conditions are present.

• No MAF sensor, ECT sensor, HO2S No. 1 or HO2S No. 2 codes are set.
• Engine coolant temperature is greater than 185°F (85°C).
• Vehicle is in closed loop fuel control.
• Mass airflow is 12-40 grams per second.
• Above conditions are met for at least 2 consecutive minutes.
• Calculated engine load is steady.
• Vehicle speed is 30-68 MPH.
• PCM determines HO2S No. 2 activity to be excessive.
• All above conditions for up to 2 additional minutes after catalyst warm-up criteria has been met.
  NOTE: Test numbers refer to numbers on diagnostic chart.

1. This chart checks for conditions that can cause 3-way catalytic converter efficiency to appear degraded. Inspect and repair exhaust system as necessary. Also inspect bank 2 sensor No. 2 pigtail and engine harness for any damage that may cause an intermittent fault in high or low oxygen sensor circuit.
2. If 3-way catalytic converter replacement is necessary, ensure another condition is not present that may have caused catalyst to become damaged (misfire, high engine oil consumption or coolant consumption, retarded spark timing or weak spark). Correct engine misfire or mechanical fault before replacing 3-way catalytic converter to avoid damaging replacement converter.