CodelampTwo codes together
P0420 and P0430 together: both converters tested weak
P0420
- In plain words
- The catalytic converter on bank 1 is not cleaning the exhaust as well as it should.
- Standard name
- Catalyst System Efficiency Below Threshold (Bank 1)
- System
- Catalytic converter and oxygen sensors
P0430
- In plain words
- The catalytic converter on bank 2 is not cleaning the exhaust as well as it should.
- Standard name
- Catalyst System Efficiency Below Threshold (Bank 2)
- System
- Catalytic converter and oxygen sensors
General information, not a mechanic's diagnosis: have a mechanic confirm before any repair.
What the computer saw
P0420 is the converter on bank 1 and P0430 the converter on bank 2. Only engines with two banks, such as a V6 or V8, can set both. Both at once points either to converters that wore out together or to a cause both sides share.
Likely causes, most common first
- Both converters worn with age. They see the same miles and the same exhaust.
- An engine problem that harmed both. Misfires, burning oil or a rich mixture affect the whole engine. Fix these first.
- Faulty rear oxygen sensors. Less likely on both sides at once, but possible on an older car.
- Exhaust leaks. A leak near each rear sensor, or at a shared joint.
What you can note yourself
- Read every code. A misfire or rich code beside these changes the order of repairs.
- Note any oil use between changes, and any rattle from under the car.
What to tell the shop
- All the codes, including pending ones.
- Ask what caused both to fail before quoting two converters.
This page gives no verdict on whether a car is safe to drive; a mechanic can.