P0420Catalyst System Efficiency Below Threshold (Bank 1)
Drive normally — check soon
What it means
Your engine computer watches the oxygen sensors before and after the catalytic converter. When the rear sensor starts seeing the same swings as the front one, the converter is no longer storing and burning off what the engine leaves behind, so the computer flags it as below efficiency. Bank 1 is the side of the engine containing cylinder 1 — on a four-cylinder engine, it's the only bank.
Is it safe to drive?
Drive normally — check soon
The car drives normally and you're not doing sudden damage by using it. It will fail an emissions test until it's resolved, and if the converter was cooked by a misfire or rich mixture, that root cause needs fixing before it takes a new converter out too. If P0420 shows up alongside misfire codes, treat the misfire first — that one is urgent.
Common causes
Most likely first.
- A worn-out catalytic converter — the most common cause on higher-mileage cars
- A lazy or failing downstream O2 sensor giving a false reading
- An exhaust leak ahead of or around the sensors letting outside air in
- An engine problem upstream — misfires or a rich mixture — that overheated or fouled the converter
- Oil or coolant being burned (worn rings, a leaking head gasket) and coating the catalyst
What the live data shows
The tell is in the two O2 sensors on bank 1. Upstream (Bank 1 Sensor 1) should oscillate steadily between roughly 0.1 and 0.9 V; downstream (Bank 1 Sensor 2) should hold comparatively flat. With a failing catalyst, the downstream voltage starts mirroring the upstream swings. Catalyst temperature and the bank 1 fuel trims help separate a genuinely dead converter from a sensor or mixture problem.
A scanner that reads all 139 standard sensors shows you these readings live. ScanMyCar charts them as you drive, keeps the freeze-frame stored with the code, and pulls codes from every module — free, with any cheap Bluetooth adapter.
Typical fixes
Cheapest first, where honesty allows.
- Check for exhaust leaks and inspect the downstream O2 sensor's wiring — the cheap possibilities
- Fix any misfire, fuel-trim, or oil-burning problem before condemning the converter
- Replace a lazy downstream O2 sensor if its readings are implausible
- Replace the catalytic converter — insist on an OEM-grade unit; bargain universal cats often set P0420 again
A guide, not a diagnosis. This page describes what P0420 generally means across OBD-II cars. What's actually wrong with your car depends on its live data and its history — and any repair decision deserves a proper diagnosis, whether that's you with a scanner or a professional with a lift.