OBD codes / P0420
P0420
ModerateCatalyst System Efficiency Below Threshold (Bank 1)
You can usually keep driving with P0420 in the short term — it rarely leaves you stranded. But it will not pass an emissions or inspection test, and if a failing converter is the cause it can slowly worsen and hurt performance and fuel economy. Get the cause confirmed before buying parts, since a cheap sensor and an expensive converter set the identical code.
What this code means
Your catalytic converter cleans the exhaust by turning harmful gases into less harmful ones. Two oxygen sensors — one before the converter and one after — let the computer check that it is working: in a healthy system the downstream sensor reads much steadier than the upstream one. P0420 sets when those two readings look too similar on Bank 1, meaning the converter is not reducing emissions efficiently enough. It most often points to a worn-out converter, but a lazy or faulty oxygen sensor, an exhaust leak, or an unaddressed engine misfire can trigger the very same code.
Symptoms you might notice
- Often no change in how the car drives — just the check-engine light
- Failed emissions or smog test
- Slightly reduced fuel economy
- A faint rotten-egg (sulfur) smell from the exhaust
- Sluggish acceleration if the converter is clogged
What it costs to fix
Typical range: $150–$2,500 · about ~0.5–2.5 hrs of labor
| If the cause is… | Part | Labor |
|---|---|---|
| Downstream oxygen (O2) sensor | $50–$300 | ~0.5–1.5 hrs |
| Catalytic converter | $400–$2,400 | ~1–2.5 hrs |
| Exhaust leak repair (gasket or pipe section) | $20–$200 | ~0.5–2 hrs |
The big swing is whether it turns out to be just an oxygen sensor versus the catalytic converter itself. Converters on luxury, hybrid, or newer low-emission vehicles — and genuine OEM parts — can run well above these figures.
The price swings on which cause it turns out to be — so confirm the cause before paying. Diagnose P0420 for my exact vehicle →
Frequently asked
Can I drive with P0420?
In most cases yes, at least short-term — it is not typically an immediate safety risk. The downsides are failing an emissions test and, if the catalytic converter is degrading, gradually worse performance and fuel economy. It is fine to drive to your mechanic, but do not ignore it long-term.
Will my car pass inspection with P0420?
No. In areas with emissions or smog testing, an active P0420 — and the check-engine light it turns on — is an automatic fail. You will need to fix the underlying cause and clear the code first.
Is it the oxygen sensor or the catalytic converter?
Both set the same code, and that is exactly the problem: replacing a $1,500 converter when it is really a $200 sensor wastes money, and vice versa. A proper diagnosis — checking the sensor data and ruling out exhaust leaks and misfires — tells you which before you buy any parts.
Will P0420 clear itself?
Sometimes the light goes off briefly if the converter is borderline, but the code almost always comes back until the real cause is fixed. Clearing it without a repair will not help you pass inspection.
How urgent is P0420?
Low-to-moderate. It is safe to keep driving briefly, but have it diagnosed soon — a clogging converter can eventually restrict the exhaust and sap power, and an unaddressed misfire or oil problem can ruin a new converter.
Seeing P0420 on your car? Get a diagnosis specific to your exact year, make and model.
Diagnose P0420 for my vehicleGeneric OBD-II reference. Manufacturer-specific behavior varies — confirm with a scan tool and, for safety-related codes, a professional inspection.