P0420 Code: Catalyst System Efficiency Below Threshold (Bank 1)
P0420 means the engine computer has detected that the catalytic converter on bank 1 is working below its required efficiency. The cause is often a worn converter, but a faulty downstream oxygen sensor, an exhaust leak or an engine misfire can trigger the same code. Diagnose before replacing the converter, which is the most expensive fix.
What P0420 means
Your car has an oxygen sensor before the catalytic converter (upstream) and one after it (downstream). A healthy converter stores and uses oxygen, so the downstream sensor shows a steady signal while the upstream one switches rapidly. When the downstream signal starts to mirror the upstream one, the computer concludes the converter is no longer doing its job and sets P0420.
"Bank 1" is the side of the engine with cylinder 1. On 4-cylinder engines there is only one bank. The same fault on bank 2 of a V6 or V8 is code P0430.
Symptoms
- Check engine light (often the only symptom)
- Failed emissions or smog inspection
- Rotten-egg smell from the exhaust
- Sluggish acceleration if the converter is clogged
- Slightly worse fuel economy
Common causes, from cheapest to most expensive
| Cause | How to spot it | Typical fix |
|---|---|---|
| Exhaust leak before or near the rear sensor | Ticking or puffing sound, soot marks on joints | Gasket or clamp |
| Faulty downstream oxygen sensor | Sensor data stays flat or slow on a scan tool | Oxygen sensor replacement |
| Engine misfire or rich running | Misfire codes, fuel trim codes, rough idle | Fix the misfire first |
| Oil or coolant burning | Blue or white smoke, low oil or coolant | Engine repair |
| Worn catalytic converter | Confirmed after ruling out the above | Converter replacement |
How to diagnose P0420 properly
Many shops charge a diagnostic fee, often one hour of labor. It is worth paying: replacing a converter that is not actually bad is one of the most expensive mistakes on this code.
- Read all stored codes: misfire or fuel trim codes must be fixed first
- Watch upstream and downstream oxygen sensor data with a scan tool at a steady 2,500 rpm
- Inspect the exhaust for leaks between the engine and the downstream sensor
- Check that the engine reaches normal temperature (a stuck-open thermostat can affect readiness)
- Only then condemn the converter
Can you drive with P0420?
Usually yes, in the short term: P0420 alone rarely causes drivability problems. But you will fail emissions testing, and if the underlying cause is a misfire, continued driving can damage the converter further. If the check engine light is flashing, that is a different and more urgent problem.
Before you pay: warranty and recalls
Federal law requires an emissions warranty of 8 years or 80,000 miles on the catalytic converter for most vehicles, and California-certified cars often have longer coverage. Check your warranty booklet, and use our free recall check in case your model has a recall or extended warranty for this code.
Frequently asked questions
Can a bad oxygen sensor cause P0420?
Yes. A slow or failing downstream oxygen sensor can make a good converter look bad. It is a much cheaper fix, so it should be checked first.
Will a catalytic converter cleaner fix P0420?
Rarely for good. Additives may help a marginal converter pass a test, but they do not repair a worn or damaged one.
Can I clear the code and pass inspection?
No. Clearing the code also resets the readiness monitors, and most inspections fail a car whose monitors are not ready. The code will return if the fault is still there.
What is the difference between P0420 and P0430?
They mean the same thing on different sides of the engine: P0420 is bank 1, P0430 is bank 2.
Related repair costs
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Sources and methodology
Estimates combine typical book labor times, current aftermarket and OEM parts prices, and 2026 US shop labor rates. See exactly how we calculate them.
- U.S. Bureau of Labor Statistics: Occupational Employment and Wage Statistics, automotive service technicians (49-3023), by state
- U.S. Bureau of Labor Statistics: Automotive Service Technicians and Mechanics (pay and duties)
- Federal Trade Commission: Auto Repair Basics (estimates, warranties, your rights)
- NHTSA: Check for recalls before paying for a repair (our free recall check uses NHTSA data)
- ASE (National Institute for Automotive Service Excellence): technician certification