OBD-II DTC · POWERTRAIN · Severity: MEDIUM
P0420: Catalyst System Efficiency Below Threshold (Bank 1)
Oxygen sensor 2 (downstream) waveform mirrors upstream O2 sensor, indicating reduced oxygen storage capacity in the three-way catalytic converter.
| DTC Code | P0420 |
|---|---|
| Standard Definition | Catalyst System Efficiency Below Threshold (Bank 1) |
| Subsystem | POWERTRAIN |
| Urgency / Severity | MEDIUM |
| Standard Reference | SAE J2012 / ISO 15031-6 |
1. Symptoms
- Check Engine Light (MIL) illuminated
- Sulfur or rich exhaust odor
- Failed emissions / OBD readiness test
2. Common Root Causes
- Aged or thermally degraded three-way catalytic converter
- Exhaust manifold or flex-pipe leak upstream of rear O2 sensor
- Lazy or contaminated downstream oxygen sensor (Bank 1 Sensor 2)
- Engine misfire or fuel trim imbalance poisoning catalyst
3. Diagnostic Procedure
- Compare Bank 1 Sensor 1 and Sensor 2 live voltage graphs at 2,000 RPM warm cruise
- Inspect exhaust system for leaks between manifold and rear sensor
- Verify long-term fuel trims (LTFT) are within ±8% and no misfire codes are stored