P0421 - WARM-UP CATALYST EFFICIENCY BELOW THRESHOLD (BANK 1)

The Engine Control Module (ECM) observes the switching frequency ratio of the front and rear oxygen (O2) sensors. The Three-Way Catalyst (TWC) can store oxygen and will indicate a low oxygen sensor 2 switching frequency.

When the oxygen storage capacity is reduced, the switching frequency of the O2 (Oxygen Sensor) will increase. If the frequency ratio of the oxygen sensors reaches the specified limit value, the fault code P0421 OBD2 is set, indicating a malfunction of the catalytic converter (TWC).

• You probably don't have any driving problems, but the Check Engine service light comes on in the dashboard.

When the diagnostic code P0421 OBD II is set, one or more of the following problems have likely occurred:

• Some spark plugs may be dirty or defective.
• There may be leaks in the exhaust system.
• Some cable from one of the oxygen sensors may be in bad condition.
• One or both of the oxygen (O2) sensors may not work properly.
• Perhaps, the TWC (Catalytic Converter) is failing.

To solve the code P0421, you can do the following:

• Check that the spark plugs are in good condition. Change them if necessary.
• Make sure there's a good connection between the oxygen sensors. Repair it if you find inconsistency in one of the connections.
• Measure the voltage of the oxygen (O2) sensors and verify that it is within the required parameters.