CarSpecFix
OBD-II trouble code

P0420: Catalyst System Efficiency Below Threshold (Bank 1), causes & can you keep driving?

Quick answer

What it means
Catalyst System Efficiency Below Threshold (Bank 1)[1]
Wording from Subaru Service Information Bulletin, DTC P0420 Diagnostic Procedure, p.1
Can I keep driving?
It is important to note that an illuminated dashboard light, as described here, is intended to inform the driver of the need for service, NOT of the need to stop the vehicle. However, service should be sought as soon as possible.[2]
U.S. EPA Fact Sheet EPA 420-F-97-003 (May 1997), Frequently Asked Questions About On-Board Diagnostics, p.2 · applies to all OBD-II vehicles
Under certain conditions, the dashboard light will blink or flash. This indicates a rather severe level of engine misfire. When this occurs, the driver should reduce speed and seek service as soon as possible.[2]
U.S. EPA Fact Sheet EPA 420-F-97-003 (May 1997), Frequently Asked Questions About On-Board Diagnostics, p.2 · applies to all OBD-II vehicles
Toyota only Malfunction indicator lamp ... Indicates a malfunction in: The electronic engine control system; The electronic throttle control system; or The electronic automatic transmission control system → Have the vehicle inspected by your Toyota dealer immediately.[3]
How urgent?
Not rated
Severity is rated only when a source states it.
Shop visit or check it yourself first?
Not yet verified — source pending
Last verified 2026-10-15Report a data error

Possible causes

Each cause comes from a manufacturer document and applies to the models listed. Your vehicle may differ.

  1. Subaru's service bulletin (Subaru SIB P0420, p.1-2) [1]
    Other engine/O2/fuel DTCs must be fixed first; then mode 6 catalyst test before replacement
    Seen on: 2017-19MY Legacy & Outback 2.5L; 2020-23MY Legacy & Outback; 2017-23MY Impreza & Crosstrek; 2017-18MY Forester 2.5L; 2019-23MY Forester; 2017-23MY WRX; 2019-21MY Ascent
  2. Honda's service bulletin (Honda Service Bulletin 22-040, p.1) [4]
    Sensors misinterpret catalyst as deteriorated, or catalyst actually deteriorated; PGM-FI software update
    Seen on: 2014-16 Honda CR-Z
  3. Ford's service bulletin (Ford TSB 22-2260, p.1) [5]
    High sulfur content in fuel; desulfation procedure
    Seen on: 2018-20 F-150 2.7L, 2020-21 Expedition/Navigator
  4. Ford's service bulletin (Ford TSB 25-2119, p.1) [6]
    PCM software
    Seen on: 2021-2022 F-Super Duty 7.3L, Pickup only

Sources

  1. Subaru Service Information Bulletin, DTC P0420 Diagnostic Procedure, p.1, retrieved 2026-10-15
  2. U.S. EPA Fact Sheet EPA 420-F-97-003 (May 1997), Frequently Asked Questions About On-Board Diagnostics, p.2, retrieved 2026-10-15
  3. 2022 Toyota Camry Owner's Manual OM06244U, p.528, retrieved 2026-10-15
  4. Honda Service Bulletin 22-040, p.1, retrieved 2026-10-15
  5. Ford TSB 22-2260, p.1, retrieved 2026-10-15
  6. Ford TSB 25-2119, p.1, retrieved 2026-10-15

General information from manufacturer documents, not professional mechanical advice. See Data sources.