Symptom hub
P0420 But the Car Runs Fine: Why That Does Not Automatically Mean You Need a Converter
A car can drive almost normally with P0420 because catalyst efficiency is often an emissions-monitor problem before it becomes a true drivability problem. The key is separating a genuinely weak converter from an exhaust leak, an O2-monitor story, or leftover aftermath from a misfire or rich-running event that already happened.
Likely related codes
Start your diagnostic path here
P0300 — Random/Multiple Cylinder Misfire Detected P0171 — System Too Lean (Bank 1) P0172 — System Too Rich (Bank 1) P0420 — Catalyst System Efficiency Below Threshold (Bank 1) P0430 — Catalyst System Efficiency Below Threshold (Bank 2) P0136 — O2 Sensor Circuit (Bank 1 Sensor 2) P0137 — O2 Sensor Circuit Low Voltage (Bank 1 Sensor 2) P0138 — O2 Sensor Circuit High Voltage (Bank 1 Sensor 2) P0156 — O2 Sensor Circuit (Bank 2 Sensor 2) P0157 — O2 Sensor Circuit Low Voltage (Bank 2 Sensor 2) P0158 — O2 Sensor Circuit High Voltage (Bank 2 Sensor 2) P0139 — O2 Sensor Circuit Slow Response (Bank 1 Sensor 2) P0140 — O2 Sensor Circuit No Activity Detected (Bank 1 Sensor 2) P0159 — O2 Sensor Circuit Slow Response (Bank 2 Sensor 2) P0160 — O2 Sensor Circuit No Activity Detected (Bank 2 Sensor 2) P0421 — Warm Up Catalyst Efficiency Below Threshold (Bank 1) P0431 — Warm Up Catalyst Efficiency Below Threshold (Bank 2) P2096 — Post Catalyst Fuel Trim System Too Lean (Bank 1) P2097 — Post Catalyst Fuel Trim System Too Rich (Bank 1) P2098 — Post Catalyst Fuel Trim System Too Lean (Bank 2) P2099 — Post Catalyst Fuel Trim System Too Rich (Bank 2) P2270 — O2 Sensor Signal Stuck Lean (Bank 1 Sensor 2) P2271 — O2 Sensor Signal Stuck Rich (Bank 1 Sensor 2) P2272 — O2 Sensor Signal Stuck Lean (Bank 2 Sensor 2) P2273 — O2 Sensor Signal Stuck Rich (Bank 2 Sensor 2) P2430 — Secondary Air Injection System Air Flow / Pressure Sensor Circuit P2431 — Secondary Air Injection System Air Flow / Pressure Sensor Circuit Range / Performance P2432 — Secondary Air Injection System Air Flow / Pressure Sensor Circuit Low P2433 — Secondary Air Injection System Air Flow / Pressure Sensor Circuit High P2440 — Secondary Air Injection System Switching Valve Stuck Open Bank 1 P2441 — Secondary Air Injection System Switching Valve Stuck Closed Bank 1 P2442 — Secondary Air Injection System Switching Valve Stuck Open Bank 2 P2443 — Secondary Air Injection System Switching Valve Stuck Closed Bank 2