Catalytic converter codes
A catalyst code such as P0420 means the computer judged the catalytic converter’s efficiency below its limit. The monitor uses sensors before and after the converter, so a sensor, an exhaust leak or an engine problem can affect the result as well as the converter itself.
Bank and sensor
Catalyst and oxygen sensor codes use two labels. Bank 1 is the side of the engine with cylinder 1. Bank 2 is the other side, and a four-cylinder engine only has bank 1. Sensor 1 sits ahead of the converter. Sensor 2 sits after it. So P0420 is about bank 1, and a code for bank 1 sensor 2 is about the sensor behind that converter.
Fix what damages converters first
A misfire sends unburned fuel into the converter, where it burns and overheats it. That’s why a flashing light calls for slowing down. A rich-running engine can do similar harm. If the scan shows misfire or fuel-trim codes with the catalyst code, those come first.
Ask what tests point to the converter
Before quoting a converter, a shop should check for exhaust leaks near the sensors, review the upstream and downstream sensor signals with the engine warm, and confirm there are no active misfires. If the converter really has failed, ask what caused it.
Codes that appear together
P0420 and P0430 together point toward something both banks share, such as the fuel mixture. Post-catalyst fuel-trim codes P2096 through P2099 come from the same downstream sensors. Volkswagen’s bulletin 01-20-01 for 2018–2019 Atlas 3.6L models lists all of these together and replaces the converter with updated software.
Buying a replacement converter
Federal rules require an aftermarket converter to be the right type for the vehicle and to carry an EPA-compliant label. New aftermarket converters carry a 5-year or 50,000-mile warranty on the shell and a 25,000-mile performance warranty. In California, an aftermarket converter needs a CARB executive order that covers your vehicle, and CARB’s aftermarket parts database lists them.
Before paying for one, ask whether your vehicle is still covered by the manufacturer’s emissions warranty.