Technical Definition:

Warm Up Catalyst Efficiency Below Threshold (Bank 1)

Trouble Code: P0421
Address: 0x0421
System: Powertrain
Technical Description:

Basically this means that the oxygen sensor downstream of the catalytic converter on bank 1 is detecting that the converter is not working as efficiently as it should be (according to specs). It is part of the vehicle emissions system.

Possible Causes:

Possible Symptoms:

Warm Up Catalyst Efficiency Below Threshold Bank 1

Possible Solutions:

Measure the voltage at the oxygen sensor on Bank 1 (the rear sensor, or the sensor after the converter). In fact, it would be a good idea to test each oxygen O2 sensor while you’re at it.

One thing to note is that many vehicle manufacturers offer a longer warranty on emissions-related parts. So if you have a newer car but it’s out of it’s bumper-to-bumper warranty, there still may be warranty on this type of problem. Many manufacturers give a five year, unlimited mileage warranty on these items. It’s worth checking into.

OBD-II Trouble Code Type:

Diagnostic Step:

Tech Notes:

– Warm Up Three Way Catalytic converter deterioration or malfunction
– Exhaust system leaks
– Loose front or rear HO2Ss
– Faulty front heater O2 sensor
– O2 sensors harness is open or shorted
– O2 sensors poor electrical connection

Specific Makes:

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