P0420

Catalyst System Efficiency Below Threshold (Bank 1)

🟡 SERVICE SOON POWERTRAIN · Catalyst System

Downstream O2 sensor on Bank 1 is switching too actively — indicates cat is no longer storing oxygen efficiently.

What You'll Notice
  • MIL on, often no drivability complaint
  • Faint sulfur odor from exhaust in rare cases
  • Borderline emissions test failure
Probable Causes
  • Catalytic converter degraded — substrate crumbling after high-mileage use
  • Previous misfires overheated and melted the cat substrate
  • Oil or coolant burning through the cat (check for other DTCs: P0301–P0308, P0302)
  • Leaking exhaust manifold gasket upstream of B1S2 O2 sensor
  • Faulty downstream O2 sensor (B1S2) — diagnose before replacing cat
Direct Repair Steps
  1. Monitor B1S1 and B1S2 O2 sensor waveforms live; if B1S2 mirrors B1S1 symmetry, cat is dead
  2. Check for misfires or oil consumption first — address root cause before swapping cat
  3. Replace OEM or OEM-equivalent catalytic converter; universal cats often fail emissions
  4. Clear code, drive two full drive cycles, rescan — confirm no recurrence
  5. Replace B1S2 oxygen sensor if waveform looks abnormal but cat tests healthy
Vehicles Affected
toyota honda chevy ford subaru hyundai lexus
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