P0420: Catalyst System Efficiency Below Threshold (Bank 1)
Your catalytic converter isn't cleaning exhaust gases as well as it should. Most often the converter itself is aging out, but a lazy downstream oxygen sensor or a small exhaust leak can trigger the same code.
Can I keep driving?
Usually yes. The car normally runs fine, but it will fail an emissions test, and a converter that keeps degrading can eventually restrict exhaust flow.
Symptoms you might notice
- Check engine light on, often with no change in how the car drives
- Failed emissions or smog test
- Sometimes a rotten-egg smell from the exhaust
What to check first
- Look and listen for an exhaust leak ahead of the converter. A ticking or hissing sound on a cold start is a clue.
- Check for other stored codes. Misfire or fuel-trim codes can make a healthy converter look bad, so fix those first.
- With a scan tool, watch the upstream and downstream oxygen sensors. The downstream one should read steadier than the upstream one. If it mirrors it, the converter is the likely problem.
- Only after those checks, decide between the downstream sensor and the converter. Replacing the converter first is the expensive guess.
Most common causes
- Worn or failing catalytic converter
- Faulty downstream (post-cat) O2 sensor
- Exhaust leak before or at the converter
- Engine running rich from upstream fuel/ignition problem
Check it against your exact vehicle
A code means the same thing on every car, but where the parts are and what to check first depends on your year, make, model and engine. Tell CaliperAI what you are seeing and it will help narrow down the likely causes for your vehicle.
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General information about this code, not a diagnosis of your specific vehicle. Safety-critical work such as brakes, fuel and airbags is best left to a qualified mechanic.