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P0420 Catalyst system efficiency below threshold, bank 1

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Catalyst system efficiency below threshold, bank 1, readable through chat.

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System
Emissions control
Component
Catalytic converter

Symptoms

  • Check engine light on
  • Engine stalls
  • Idle mixture out of specification
  • Reduced fuel economy
  • Sulfur or rotten egg smell
  • Rattle from the exhaust
  • Failed emissions test

Common causes

  • Exhaust leak or loose joint near the converter or sensors
  • Worn or damaged catalytic converter
  • Failing rear oxygen sensor
  • Water or a wiring fault at the rear oxygen sensor connector
  • Engine oil use
  • Ignition fault
  • Fuel or intake fault

Guides for this code

Pencil sketch of a Subaru Forester in a garage
Diagnostics & Repair

P0420 on a 2014-2015 Subaru Forester 2.5L: Causes and Repair

JC Landry · Oct 27, 2026

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