DTC P0420: Catalyst System Efficiency Below Threshold: Condition

DTC is set when value from rear HO2S becomes closer to value from front HO2S. This is monitored when engine speed is 1090-3090 RPM, vehicle speed is 16.4-64.3 MPH and vehicle is under 16-55 percent (M/T) load or 20-55 percent (A/T) load. This is an intermittent monitor. MIL is illuminated when Powertrain Control Module (PCM) detects malfunction during 2 consecutive drive cycles. Possible causes are:

• Three Way Catalytic (TWC) converter deterioration or malfunction.
• Exhaust system leak.
• Loose rear HO2S.
• Loose front HO2S.
• Front HO2S malfunction.