YANMAR-P0420 moderate Yanmar ›

Oxidation Catalyst Efficiency Below Threshold

The YANMAR-P0420 (Yanmar) diesel fault code means: Oxidation Catalyst Efficiency Below Threshold. This is a moderate severity code.

My Garage →
Keep driving?
Yes, but fix soon
DIY difficulty
advanced
Estimated cost
$1,500-$4,500

What Are the Symptoms of YANMAR-P0420?

  • Frequent regens

What Causes YANMAR-P0420? (Most Likely First)

  • Aged DOC Likely
  • Injector leak contaminating DOC Possible

Can I Still Drive with YANMAR-P0420?

Yes, but fix soon. YANMAR-P0420 is rated moderate severity. It is not an immediate stop-driving fault, but it will not clear itself — book the repair rather than ignoring it.

Frequently Asked Questions

What is Yanmar code P0420?

Yanmar P0420: Oxidation Catalyst Efficiency Below Threshold. This code is reported on Yanmar equipment running the described engine family.

Can I keep working with Yanmar code P0420?

Usually drivable short-term, but fix soon to avoid derate or DPF issues.

How much does Yanmar P0420 cost to fix?

Estimated repair cost: $1,500-$4,500. Off-road diesel shop labor runs $125-$175/hr in 2026. Dealer diagnostic software (Yanmar SmartAssist / YDT) is usually required.

Mechanics Guide Step-by-step diagnosis

How Do You Diagnose YANMAR-P0420?

  1. Measure DOC temp delta during regen.

  2. Inspect injectors for copper washer failure.

Sources 1

This page is built from documented references. Verify against your own service info before repair work.

  • SAE J2012Diagnostic Trouble Code Definitions (defines P0420 as a generic OBD-II code)

P0420 on Other Platforms

The same code ID appears across other engines, vehicles, and equipment. Diagnostic flow varies by platform — see the matching breakdown:

Gas / Automotive (OBD-II) Generic OBD-II definition for P0420 Bobcat moderate Oxidation Catalyst Efficiency Below Threshold Duramax serious Catalyst System Efficiency Below Threshold Bank 1 Hino serious Catalyst System Efficiency Below Threshold (DOC) Isuzu serious Catalyst System Efficiency Below Threshold (DOC) PACCAR serious Catalyst System Efficiency Below Threshold Bank 1

Which Vehicles Commonly Get YANMAR-P0420?

  • Yanmar SA221 / SA324 / SA424 compact tractors
  • Yanmar YT235 / YT347 / YT359 compact tractors
  • Yanmar LX4100 / LX4900 utility tractors
  • Yanmar ViO17 / ViO25 / ViO35 / ViO45 / ViO55 / ViO80 mini excavators
  • Yanmar B27-3 / B37-3 / B50-3 compact excavators
  • Cub Cadet Yanmar Ex3200 / Lx410 compact tractors (rebadged)
  • John Deere 17G / 26G / 35G compact excavators (Yanmar powered)
  • Takeuchi TB210R / TB216 / TB230 mini excavators (Yanmar powered)

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