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P0420 Catalyst System Efficiency Below Threshold

Diagnostic Fault Code P0420
Low performance of the catalyst system

The three-function catalyst limits the emission of hydrocarbons, carbon monoxide (CO) and nitrous oxide (NOx). The catalyst in the converter triggers the chemical reaction of the oxidation of hydrocarbons and carbon oxides present in the exhaust gas. In this process, the hydrocarbons and carbon monoxide are decomposed into water vapor and carbon dioxide (CO2) as well as the NOx reduction resulting in the formation of nitrogen. The catalyst also stops the oxygen. The engine control module (ECM) monitors this process using heated oxygen sensors (HO2S) in the exhaust gas stream before and after the three-way catalytic converter. HO2S produces an output signal that the ECM uses to calculate the oxygen storage capacity of the catalyst. This is an indication of the catalyst’s ability to effectively utilize the exhaust gas. The ECM monitors the efficiency of the catalyst by monitoring HO2S at the fuel cut-off at engine speed reduction (DFCO) when operating above idle. When the catalyst is working properly, the HO2S probe behind the catalyst for feeding the fuel during the DFCO is free compared to the HO2S response of the catalyst. If the HO2S probe behind the catalyst and the HO2S probes in front of the catalyst are close, this may mean that the oxidation ability and efficiency of the catalyst have fallen below the acceptable threshold. Fault code P0420 may apply to such car brands as Toyota, Chevrolet, Ford, Nissan, Honda, GMC, Dodge, etc.

Check for the following conditions that may cause a decrease in catalyst performance:

• Ignition interruption
• High consumption of engine oil or coolant
• Delayed timing phases
• Poor or small spark
• Poor fuel mixture
• A rich fuel mix
• Defective lambda sensor or wiring harness

Check the following conditions for the catalyst:

  • If it is not crushed
  • Strong discoloration caused by excessive temperatures
  • Damages from the road surface
  • External rattling due to damage to the catalyst substrate
  • Flow restrictions
    If a fault is detected
    Replace the catalyst.


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