What are the symptoms of P0420?

MIL on, often no drivability complaint. Faint sulfur odor from exhaust in rare cases. Borderline emissions test failure

Monitor B1S1 and B1S2 O2 sensor waveforms live; if B1S2 mirrors B1S1 symmetry, cat is dead. Check for misfires or oil consumption first — address root cause before swapping cat. Replace OEM or OEM-equivalent catalytic converter; universal cats often fail emissions. Clear code, drive two full drive cycles, rescan — confirm no recurrence. Replace B1S2 oxygen sensor if waveform looks abnormal but cat tests healthy

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

Monitor B1S1 and B1S2 O2 sensor waveforms live; if B1S2 mirrors B1S1 symmetry, cat is dead
Check for misfires or oil consumption first — address root cause before swapping cat
Replace OEM or OEM-equivalent catalytic converter; universal cats often fail emissions
Clear code, drive two full drive cycles, rescan — confirm no recurrence
Replace B1S2 oxygen sensor if waveform looks abnormal but cat tests healthy

Vehicles Affected

toyota honda chevy ford subaru hyundai lexus