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

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Cost of diagnosing the P0423 code

P0423 is a diagnostic trouble code (DTC) for "Heated Catalyst Efficiency Below Threshold (Bank l)". This can happen for multiple reasons and a mechanic needs to diagnose the specific cause for this code to be triggered in your situation. Our certified mobile mechanics can come to your home or office to perform the Check Engine Light diagnostic for $70.00. Once we are able to diagnose the problem, you will be provided with an upfront quote for the recommended fix and receive $30.00 off as a credit towards the repair. All our repairs are backed by our 12-month / 12,000-mile warranty.

Cars Estimate Credit towards follow-up repair Earliest Availability
Ford$70.00$30.00
Toyota$70.00$30.00
BMW$70.00$30.00
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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?

YourMechanic offers certified mobile mechanics who will come to your home or office to diagnose and repair your vehicle. Get a quote and book an appointment online or speak to a service advisor at 1-800-701-6230.

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