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P0420 Code — Meaning, Real Fixes & Costs | MyCarFault
P0420
🟡 Medium Engine system

P0420 — Catalyst System Efficiency Below Threshold B1

12 Driver reports
12 Solved cases
6 Verified fixes
0 Wrong repairs reported

📊 Diagnostic Overview

Severity, cost and difficulty for P0420, based on data already on this page.

Severity
🟡 Medium
Diagnostic Difficulty
Moderate
System
Engine
Common Causes (based on confirmed repairs)
  • Before buying a new cat: 1) Fix any misfires first — P0300/P030x codes damage the cat. 2) Check exhaust leaks before the converter. 3) Test downstream O2 sensor — a bad sensor is $50-150 vs $500+ for a new cat. 4) Try catalytic converter cleaner additive. 1 vehicle
  • Downstream O2 sensor replaced first $85. Code returned 1000 miles later. Catalytic converter genuinely worn — replaced with quality aftermarket $320. Passed emissions test. 226 vehicles
  • Added one bottle of catalytic converter cleaner (CRC brand) per instructions. Drove 200 miles highway. Code cleared and stayed off for 8000 miles. Second bottle at 8000 miles — still clean at 20000 miles total. Genuine mild carbon buildup case — cleaner w 1 vehicle
  • Checked downstream O2 sensor first — reading was sluggish. Replaced rear O2 sensor $85. Code cleared and did not return after 500 miles. 1 vehicle
  • Downstream O2 sensor replaced first $95. Code returned after 500 miles. Cat confirmed bad — replaced with quality aftermarket $280. Code has not returned in 15000 miles. 1 vehicle

📐 Diagnostic Difficulty reflects how many different repairs drivers have tried for P0420 and how often those attempts turned out to be the wrong fix — it is calculated from MyCarFault's own reports, not an official manufacturer rating.

⭐ MOST VERIFIED FIX FOR P0420
Before buying a new cat: 1) Fix any misfires first — P0300/P030x codes damage the cat. 2) Check exhaust leaks before the converter. 3) Test downstream O2 sensor — a bad sensor is $50-150 vs $500+ for a new cat. 4) Try catalytic converter cleaner additive.
Worked in 0% of voted outcomes · Verified by 0 drivers
⚪ Unverified

Most Verified Fixes for P0420

Ranked by confidence score — same vehicle matches weighted higher.

Before buying a new cat: 1) Fix any misfires first — P0300/P030x codes damage the cat. 2) Check exhaust leaks before the converter. 3) Test downstream O2 sensor — a bad sensor is $50-150 vs $500+ for a new cat. 4) Try catalytic converter cleaner additive. 0%
0 worked · 1 total votes · ⚪ Unverified
Downstream O2 sensor replaced first $85. Code returned 1000 miles later. Catalytic converter genuinely worn — replaced with quality aftermarket $320. Passed emissions test. 226 cases
No repair votes yet · 226 vehicles · ⚪ Unverified
Added one bottle of catalytic converter cleaner (CRC brand) per instructions. Drove 200 miles highway. Code cleared and stayed off for 8000 miles. Second bottle at 8000 miles — still clean at 20000 miles total. Genuine mild carbon buildup case — cleaner w 1 case
No repair votes yet · ⚪ Unverified
Checked downstream O2 sensor first — reading was sluggish. Replaced rear O2 sensor $85. Code cleared and did not return after 500 miles. 1 case
No repair votes yet · ⚪ Unverified
Downstream O2 sensor replaced first $95. Code returned after 500 miles. Cat confirmed bad — replaced with quality aftermarket $280. Code has not returned in 15000 miles. 1 case
No repair votes yet · ⚪ Unverified
Rear O2 sensor replaced $65. Code cleared and has not returned in 18000 miles. Catalyst confirmed healthy after sensor replacement. 1 case
No repair votes yet · ⚪ Unverified

Driver Reports — P0420 12 reports

🟢 SOLVED 80% P0420 Catalyst System Efficiency Below Threshold Bank 1
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🟢 SOLVED 80% Ford Escape 2018 Catalytic converter P0420 code
Ford 2018 👁 2,848
🟢 SOLVED 80% Porsche Cayenne 2022 Catalytic converter P0420 code
Porsche Cayenne 2022 👁 2,840
🟢 SOLVED 80% Chevrolet Colorado 2021 Catalytic converter P0420 code
2021 👁 2,835
🟢 SOLVED 80% Honda CR-V 2013 Catalytic converter P0420 code
Honda 2013 👁 2,832
🟢 SOLVED 80% Dodge Ram 1500 2021 Catalytic converter P0420 code
2021 👁 2,826
🟢 SOLVED 80% Land Rover Freelander 2018 Catalytic converter P0420 code
Land Rover Freelander 2018 👁 2,793
🟢 SOLVED 80% Ford Mustang 2018 Catalytic converter P0420 code
Ford Mustang 2018 👁 2,790
🟢 SOLVED 80% Nissan Leaf 2020 Catalytic converter P0420 code
Nissan Leaf 2020 👁 2,787
🟢 SOLVED 80% Chevrolet Equinox 2019 Catalytic converter P0420 code
2019 👁 2,784
🟢 SOLVED 80% Volvo S60 2021 Catalytic converter P0420 code
Volvo 2021 👁 2,773
🟢 SOLVED 80% Mercedes-Benz CLA 2017 Catalytic converter P0420 code
Mercedes-Benz 2017 👁 2,764

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