P0420
Catalyst System Efficiency Below Threshold (Bank 1)
Detailed Description
The ECM compares the upstream and downstream oxygen sensor signals and determines that the three-way catalytic converter conversion efficiency has fallen below the calibrated minimum threshold.
Possible Causes
Catalytic converter aged or poisoned (most common, especially on vehicles over 100,000 miles)
Use of leaded or substandard fuel causing catalyst poisoning
Prolonged engine misfire causing catalyst burnout
Upstream or downstream oxygen sensor fault causing misjudgment
Exhaust system leak.
Solutions & Repair Guide
This is the most commonly encountered emission fault code. First rule out oxygen sensor faults (check upstream and downstream oxygen sensor signal response); Check for exhaust pipe leaks; If sensors are confirmed normal, the catalytic converter typically needs replacement. Recommend using OEM or high-quality catalytic converters — avoid cheap aftermarket products. Using high-quality fuel regularly can extend catalytic converter life.
Disclaimer: The information above is for reference only and does not constitute professional repair advice.
Actual causes may vary depending on vehicle make, model, year, and configuration. Always consult a qualified technician for accurate diagnosis.