P0420 – Catalyst System Efficiency Below Threshold (Bank 1) is one of the most common check engine light codes on the road. It means the oxygen sensor behind your catalytic converter is seeing almost the same readings as the sensor in front of it – in other words, the converter doesn't appear to be cleaning the exhaust.
Don't replace the cat yet
A new catalytic converter costs $900–$2,500 installed. But P0420 is triggered by the sensors' comparison, not by direct inspection of the cat. That means cheaper parts can cause the exact same code:
- Downstream O2 sensor ($30–$80): a lazy or failing rear oxygen sensor is the single most common false trigger. If the code appeared suddenly with no loss of power or fuel economy, suspect the sensor first.
- Exhaust leak (often free–$50): a small leak before or around the rear sensor lets fresh air in, skewing the reading. Listen for ticking on cold start.
- Engine misfires or rich running: unburned fuel destroys converters over time. Fix any accompanying codes (P0300-series, P0171/P0172) before condemning the cat.
- The catalytic converter itself: only after ruling out the above. High-mileage cars that burn oil or had long-term misfires are genuine candidates.
How to diagnose it yourself
Plug in an OBD2 scanner with live data and watch both O2 sensors. The front sensor should oscillate rapidly; the rear should be relatively flat and steady. If the rear sensor mirrors the front one closely at cruising speed, the converter genuinely isn't storing oxygen – that points to a real cat failure. If the rear reading is erratic or the code comes back seconds after clearing, test the sensor and wiring first.
Watch the full video guide
We cover P0420 step-by-step, including live-data examples, on our YouTube channel. Not sure if a scanner works with your car? Check the OBD2 compatibility guide.