P0420: Catalyst System Efficiency Below Threshold (Bank 1)
The catalytic converter on bank 1 is not cleaning the exhaust as well as it should, based on the oxygen sensors before and after it.
What P0420 means
A catalytic converter stores oxygen and uses it to burn off leftover fuel and toxic gases. The engine computer grades it by comparing the oxygen sensors before and after it. The upstream sensor switches rapidly, while a healthy converter keeps the downstream sensor fairly steady. RepairPal explains that P0420 sets when the downstream sensor starts switching much like the upstream sensor during the catalyst test, meaning the converter has lost its oxygen storage ability. Converters usually fail because something else damaged them: misfires that overheated them, oil or coolant burning in the engine, or long periods of rich running. RepairPal also lists bad oxygen sensors and misfiring engines as causes and warns that sensors are often replaced for nothing. Shops check for exhaust leaks, test both oxygen sensors, look for misfires and oil consumption, and sometimes measure converter temperature. The federal emissions warranty covers converter defects for 8 years or 80,000 miles, so check before paying.
Common symptoms
- Check engine light on
- Failed emissions test
- Usually no change in how the car drives
- Rotten egg smell from the exhaust in some cases
- Lack of power if the converter is clogged
Most common causes
- Worn out catalytic converter
- Faulty upstream or downstream oxygen sensor
- Engine misfire that overheated the converter
- Exhaust leak near the sensors
- Oil or coolant burning and coating the converter
- Engine running rich for a long time
What to check yourself first
Check for other codes first, especially misfire codes, and fix those before worrying about the converter. Look for exhaust leaks in front of the downstream sensor. Check whether your vehicle is still under the federal emissions warranty, which covers defects in major emission parts like the catalytic converter for 8 years or 80,000 miles.
Typical fixes and what they cost
| Fix | Typical cost | Note |
|---|---|---|
| Replace catalytic converter | $2,280-$2,480 | A real converter failure is the most expensive fix. Make sure the root cause is fixed first. |
| Replace oxygen sensor | $440-$523 | RepairPal lists oxygen sensors as a common misdiagnosis, so test before replacing. |
| Check engine light diagnosis at a shop | $125-$183 | Confirms the converter is truly bad and finds what killed it. |
| Replace spark plugs | $288-$399 | If misfires from worn plugs damaged the converter, fix them before installing a new one. |
Typical US price ranges for parts and labor, not quotes. Your shop should confirm the cause with a diagnostic before replacing parts.
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Related codes
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- P0300 Random/Multiple Cylinder Misfire Detected
- P0136 O2 Sensor Circuit (Bank 1 Sensor 2)
- P0138 O2 Sensor Circuit High Voltage (Bank 1 Sensor 2)
- P0171 System Too Lean (Bank 1)