DTC P0421: 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 1500-3000 RPM, vehicle speed is 17.3-74.5 MPH and vehicle is under 15-48 percent load. This is an intermittent monitor. MIL is illuminated when PCM detects malfunction during 2 consecutive drive cycles. Possible causes are:

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