CodelampToyota code
P0420 on a Toyota
- 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
On a Toyota, P0420 is the standard generic code: the catalytic converter on bank 1 tested below its threshold. The rear oxygen sensor's reading looks too much like the front sensor's.
Bank 1 on a Toyota
Four-cylinder Toyotas, such as most Corollas, Camrys and RAV4s, have only bank 1, so they set P0420 and never P0430.
On a Toyota V6 or V8, P0420 is the bank with cylinder 1 and P0430 the other bank. Both at once is its own case.
Many Toyotas use an air-fuel ratio sensor in front of the converter in place of an older oxygen sensor. A fault in it sets its own codes, so read every code that is stored.
Likely causes, most common first
- A worn catalytic converter. The most common cause at high mileage.
- A faulty rear oxygen sensor. Codes P0136 to P0141 beside P0420 point here.
- An exhaust leak before the rear sensor. Outside air upsets the reading.
- An engine problem that harmed the converter. Misfires or burning oil. Fix them first.
What you can note yourself
- Read every stored code, including any for the sensors before and after the converter.
- Note your engine: four-cylinder, V6 or V8.
What to tell the shop
- All the codes and your engine size.
- Ask whether they checked the sensor readings and looked for exhaust leaks before quoting a converter.
This page gives no verdict on whether a car is safe to drive; a mechanic can.