Catalytic converter efficiency
P0421
Warm Up Catalyst Efficiency Below Threshold (Bank 1)
The sensors before and after the catalytic converter are reading too much alike, which suggests the converter isn't cleaning the exhaust as well as it should.
Fine to drive
Can I keep driving? Yes, in most cases. It won't hurt the car in the short term, but get it fixed. A check engine light that's already on can't warn you about a new problem. It's safe to drive in the short term. It will fail an emissions test, and a badly clogged converter costs power.
What the words mean
- Bank 1 is the side of the engine with cylinder 1. A four-cylinder engine has only one bank.
Common causes
- A worn-out catalytic converter
- A failing rear oxygen sensor
- An exhaust leak near a sensor
- An engine that has been misfiring or running rich
Listed roughly from most to least likely. The same code can have different causes on different cars.
What to do
- Fix any misfire or mixture codes first. They kill converters.
- Have the rear oxygen sensor and the exhaust checked for leaks before buying a converter.
- Ask whether the converter is still under the emissions warranty. In the US it is often covered for 8 years or 80,000 miles.
General guidance. Your owner's manual wins. If a red warning light is on or the car drives differently, stop safely and get help.
Related codes
- P0420Catalyst System Efficiency Below Threshold (Bank 1)
- P0430Catalyst System Efficiency Below Threshold (Bank 2)
Track this automatically in Tagwell Auto
Tagwell Auto keeps your car's service dates and mileage, so when a code shows up you know what was done last. With a Bluetooth reader for the car's diagnostic port (called OBD-II), the phone app can read the code itself.