CodelampCame back after a repair
P0420 came back after a new catalytic converter
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
General information, not a mechanic's diagnosis: have a mechanic confirm before any repair.
What the computer saw
A new converter that fails the same test usually means one of two things: the problem that wore out the old one is still there, or the new converter or sensor does not pass the computer's test.
Likely causes, most common first
- The original cause was never fixed. Misfires, burning oil or a rich mixture wear out a new converter too, sometimes quickly.
- A replacement that is not the type made for the car. Ask the shop which converter was fitted and whether it is listed for your car.
- A faulty rear oxygen sensor. If the sensor was the real fault, a new converter does not change its reading.
- An exhaust leak. A leak at the new joints, before the rear sensor.
What you can note yourself
- Note how soon the light came back: right away, or after weeks.
- Keep the receipt and the part number of the new converter.
What to tell the shop
- When the converter was replaced and by whom.
- Any misfire, oil use or rich codes since.
- Ask whether the replacement is the type made for your car.
This page gives no verdict on whether a car is safe to drive; a mechanic can.