Catalyst efficiency · Engine code
P0420 and P0430: Catalyst System Efficiency Below Threshold
The engine computer compared the oxygen sensors before and after the catalytic converter and decided the converter isn't cleaning up the exhaust as well as it should. P0420 is bank 1; P0430 is bank 2 on V6 and V8 engines.
What this code means
The catalytic converter stores and uses oxygen to clean up exhaust gases. An oxygen sensor before the converter reads exhaust that's swinging rich and lean; a second sensor after it should read a much steadier signal if the converter is working.
When the downstream sensor starts copying the upstream one, the computer concludes the converter has lost its ability to store oxygen and sets P0420 (bank 1) or P0430 (bank 2). P0421 and P0431 are the same idea for the “warm-up” catalyst on engines that have one.
The code is a measurement, not a verdict. A failing sensor or an exhaust leak can make a healthy converter look bad, so the cheaper possibilities are worth ruling out before buying a converter.
Symptoms you may notice
- Check engine light with no change in how the car drives (most common)
- Failed emissions or smog test
- Rotten-egg (sulfur) smell from the exhaust
- Sluggish acceleration or rattling from under the car, if the converter is badly damaged
Common causes, most likely first
- Worn-out catalytic converterConverters lose efficiency with age and mileage. The most likely cause on older, high-mileage cars.
- Slow or failing downstream oxygen sensorA lazy sensor can make a good converter look bad. Much cheaper than a converter.
- Exhaust leak near the sensorsOutside air throws off the readings. Check manifold gaskets, flex pipes and joints.
- Past misfires or a rich-running engineUnburned fuel overheats and melts the converter's internals.
- Oil or coolant burningOil consumption or a leaking head gasket coats and poisons the converter. Replacing the converter without fixing this means it fails again.
What to check first
- Fix any misfire, rich or lean codes first. They can cause P0420, and they'll kill a new converter.
- Check for exhaust leaks, especially ahead of the downstream sensor. Ticking or puffing sounds at cold start are a clue.
- Have a shop compare the upstream and downstream oxygen sensor signals on a scan tool. A healthy converter shows a steady downstream reading.
- Check whether the car is burning oil or losing coolant.
- If you're replacing the converter, use one certified for your vehicle and state. Very cheap universal converters often bring the code back within months.
Typical repairs and rough cost
| Repair | Typical cost | Notes |
|---|---|---|
| Downstream oxygen sensor | $150–$400 | Worth testing before a converter. |
| Exhaust leak repair | $100–$500 | Gasket or flex pipe. |
| Catalytic converter | $900–$2,500+ | Higher in states that require CARB-certified parts, and for OEM converters. |
Rough US shop prices for parts and labor. Your cost depends on the vehicle, the parts brand and local labor rates.
Official code definitions
| Code | Applies to | Definition |
|---|---|---|
| P0420 | Generic | Catalyst System Efficiency Below Threshold Bank 1 |
| P0421 | Generic | Catalyst 1 Efficiency Below Threshold Bank 1 |
| P0430 | Generic | Catalyst System Efficiency Below Threshold Bank 2 |
| P0431 | Generic | Catalyst 1 Efficiency Below Threshold Bank 2 |
| P0420 | Audi | Inefficient Catalytic Converter(s), Defective Front or Rear Oxygen Sensor(s), Misfiring engines |
| P0430 | Audi | Defective Catalytic Converter, Internal engine damage resulting in high oil consumption and or a leaking Head Gasket has damaged the Catalytic Converter |
“Generic” codes mean the same thing on every make. Manufacturer entries are that brand’s own description. Look up any other code in the code library.
Common questions
Does P0420 always mean a new catalytic converter?
No. A slow downstream oxygen sensor or an exhaust leak can set the same code for a fraction of the cost. Have those checked first.
What's the difference between P0420 and P0430?
Same fault, different side of the engine. P0420 is bank 1 (the side with cylinder 1); P0430 is bank 2, which only exists on V-type engines.
Will a fuel additive fix P0420?
Additives rarely fix a converter that has truly worn out. They can't repair melted or oil-poisoned internals, and they won't fix a bad sensor or leak.
Related codes
This guide explains what the code usually means and how it’s typically diagnosed. Every car is different, and a code points to a system, not always a specific part. For anything affecting brakes, steering or a car that feels unsafe, have a qualified mechanic confirm before you rely on it.