P0420: Catalyst System Efficiency Below Threshold
P0420 means the car computer noticed the catalytic converter (Bank 1) is not working as efficiently as expected, based on comparing the upstream and downstream oxygen sensors.
Symptoms
Usually a steady check engine light with no obvious driving change, sometimes a slight performance drop or exhaust smell.
Common causes (most likely first)
A weak downstream oxygen sensor, an exhaust leak before the converter, an aged/failed catalytic converter, or poor fuel. Do not replace the converter before ruling out the sensor and leaks.
How to diagnose accurately
Read live oxygen-sensor data via OBD2: a switching upstream sensor with a near-steady downstream sensor = healthy converter; a downstream that mimics the upstream = low efficiency. Mowtar AI analyses these readings automatically.
FAQ
Can I drive with P0420?
Usually yes short-term, but it may signal a degrading converter and higher emissions — address it soon.
How much does the fix cost?
It varies by model: an oxygen sensor is far cheaper than a catalytic converter — so a correct diagnosis saves money.