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 →
How to fix P0420
Moderate — doable at home with common tools and patience
- Scan for other codes first
Pull all stored codes before touching anything. Misfire codes (P0300-series), lean/rich codes, or O2 sensor codes must be fixed first — an unaddressed misfire or fuel-mixture problem can set P0420 and will destroy a brand-new converter just as fast as the old one.
- Inspect the exhaust for leaks
With the engine cold, check the manifold, flex pipe, gaskets, and every joint ahead of the downstream O2 sensor. Listen for ticking on a cold start and look for black soot streaks at joints. Even a small leak lets outside air skew the sensor readings and mimic a failing converter.
- Watch the O2 sensor waveforms
With a scan tool showing live data, the upstream sensor should switch rapidly while the downstream sensor stays relatively flat and steady. If the downstream sensor mirrors the upstream one and both sensors respond crisply to throttle changes, the converter itself is the likely culprit. A lazy, slow-switching sensor points to the sensor instead.
- Replace the downstream O2 sensor if it is lazy
An aging downstream sensor that responds slowly or reads biased can set this code with a perfectly good converter. If the waveform looks sluggish, replacing this sensor is far cheaper than a converter and is worth doing before condemning the cat.
- Replace the catalytic converter
Only after ruling out misfires, leaks, and sensors should the converter be replaced. Use a converter certified for your state (California and CARB states require specific approvals), and fix whatever killed the old one first — converters rarely die of old age alone.
DIY or shop? A home mechanic with a decent scan tool can do the whole diagnostic sequence: reading codes, checking live O2 data, and inspecting for exhaust leaks. Replacing a downstream O2 sensor is also DIY-friendly if it isn't seized. Converter replacement is where many people go to a shop — the fasteners are usually rusted, welded-in converters need a torch or saw, and emissions-legal part selection matters. If the diagnosis points to the converter, getting a shop quote is reasonable.
Tools for the DIY steps: OBD-II scan tool with live data · Jack and jack stands or ramps · O2 sensor socket · Penetrating oil · Basic socket and wrench set
Not sure which step applies to your car? Get the likely cause for your exact vehicle before buying anything for P0420.
P0420 on specific vehicles
The likely causes shift by model. We’ve written up the patterns we see most:
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 vehicleSymptoms that trigger P0420
Drivers who get this code often notice one of these — each has its own likely-causes and cost guide:
Related trouble codes
Don’t have a scanner yet? A ~$25 Bluetooth adapter reads codes like P0420 from your phone — see our honest guide to the best OBD2 Bluetooth adapters.
Generic OBD-II reference. Manufacturer-specific behavior varies — confirm with a scan tool and, for safety-related codes, a professional inspection.