CodelampCame back after a repair
P0420 came back after a new oxygen sensor
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
P0420 is set when the rear sensor shows the converter is not working well. A new sensor fixes the code only when the old sensor was misreading. If the light is back, the new sensor may be reading correctly, and the converter is weak.
Likely causes, most common first
- The converter really is worn. The new sensor confirms what the old one said.
- The wrong sensor was replaced. The rear sensor (sensor 2) judges the converter. Replacing the front one does not change the test.
- An exhaust leak before the rear sensor. Outside air still upsets the reading.
- An engine problem harming the converter. Misfires or oil burning.
What you can note yourself
- Note which sensor was replaced: before or after the converter.
- Note how soon the light came back.
What to tell the shop
- Which sensor was replaced, and when.
- Ask for the converter test readings before and after the new sensor.
This page gives no verdict on whether a car is safe to drive; a mechanic can.