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P0423 OBD-II Trouble Code: Heated Catalyst Efficiency Below Threshold (Bank l)

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P0423 code definition

Heated Catalyst Efficiency Below Threshold (Bank l)

What the P0423 code means

P0423 is an OBD-II generic code that the engine control module (ECM) got input from the heated catalyst efficiency sensor for bank 1 that the catalyst is below efficiency levels programmed in the ECM.

What causes the P0423 code?

  • The bank 1 catalyst has lost its efficiency or has broken apart inside and needs replaced.

  • The air injection system is not pumping the correct amount of air or too much air into the bank 1 catalyst due to a fault in the air injection system.

  • The O2 sensor post catalyst on bank 1 is not working to specifications.

What are the symptoms of the P0423 code?

  • The Check Engine Light will be illuminated and the code is set in ECM memory and freeze frame.

  • The engine may not have any drive symptoms except a check engine light is on unless the catalyst is damaged and clogging the exhaust. Then all the symptoms of a blocked exhaust will be encountered like loss of power and misfiring.

How does a mechanic diagnose the P0423 code?

  • Scans codes and documents the freeze frame data in order to verify the problem

  • Clears the engine codes and retests the air injection system for proper operation

  • Checks the catalyst and sensor for any signs of damage or leaking joints in the exhaust

  • Scans the O2 sensors to see if the pre and post sensors are reading the same indicating a catalyst is broken apart

  • Checks the air injection pump for proper operation to the bank 1 catalyst

  • Compares bank 1 and bank 2 pre and post catalyst to isolate a bad sensor

Common mistakes when diagnosing the P0423 code

  • Not checking for proper operation of the air injection pump before replacing the catalyst or post O2 sensor.

  • Not checking the exhaust system for leaks before the O2 sensors before replacing the O2 sensor.

How serious is the P0423 code?

  • The post O2 sensor gives an indication of a problem with the catalyst and this could lead to a blocked exhaust system or damaged engine components from excessive back pressures.

  • The Check Engine Light being on causes the vehicle to fail emission testing.

  • The emission output levels may be above legal limits.

What repairs can fix the P0423 code?

  • Replacing the post O2 sensor for bank 1
  • Replacing the catalyst on bank 1
  • Repairing the wiring to the O2 sensors if they got burnt on the exhaust

Additional comments for consideration regarding the P0423 code

Code P0423 is a problem with the catalyst for bank 1 and the most common failure is the post O2 sensor not responding or slow to respond to changes in the exhaust O2 levels.

Need help with a P0423 code?

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