What P0420 means
Catalyst System Efficiency Below Threshold (Bank 1)
Common causes (general)
Before spending money on parts, verify the code with a scan tool and read freeze frame data. Freeze frame captures every sensor value at the moment the code set — this is often enough to figure out the root cause without further testing.
Manufacturer-specific descriptions
Different manufacturers use this code for slightly different failure modes. Here's what your specific make means by P0420:
| AUDI | Inefficient Catalytic Converter(s), Defective Front or Rear Oxygen Sensor(s), Misfiring engines |
| BMW | Catalyst System Efficiency Below Threshold |
| FORD | Catalyst System Efficiency Below Threshold (Bank 1) |
| HONDA | Catalyst System Efficiency Below Threshold |
| MITSUBISHI | Catalyst System Efficiency Below Threshold (Bank 1); Catalyst deterioration |
| NISSAN | Catalyst System Efficiency Below Threshold (Bank 1) |
| VOLKSWAGEN | Catalyst System Low Efficiency |
Read this code on your own car
DIY Fix My Car reads P0420 plus 25,000+ other DTC codes with manufacturer-specific descriptions. Includes freeze frame data, likely fix parts, and AI mechanic chat. Download free.