CC
CarCodeFix
P0420Medium Priority

Catalyst System Efficiency Below Threshold (Bank 1)

Category: Powertrain• Auxiliary Emission Controls

50 articles · 5 makes affected

Safety Warning

Fuel LeakCritical

Do not start engine. Keep away from sparks and flames.

Based on owner reports. Always prioritize safety and consult a qualified mechanic.

What is P0420?

P0420 indicates your catalytic converter is not cleaning exhaust gases effectively, typically triggered by a failing converter or upstream engine issues. This powertrain code is moderately serious and can affect emissions and performance, with Honda vehicles like the Accord and CR-V appearing most frequently in reports. Based on 975 repair cases, the average repair cost is $3847, ranging from $0 to $4700.

Common Causes

  • ●Failing catalytic converter (most common cause based on code definition)
  • ●Upstream oxygen sensor malfunction
  • ●Exhaust leaks before the catalytic converter
  • ●Engine misfires or rich/lean fuel mixture affecting converter efficiency

Symptoms You May Notice

  • ●Check Engine Light illuminated (primary symptom)
  • ●Reduced fuel economy reported by some owners
  • ●Possible lack of power during acceleration
  • ●No noticeable symptoms in some cases beyond the warning light

How to Fix It

  • ●Catalytic converter replacement ($2000-$4700, high difficulty due to cost and installation)
  • ●Oxygen sensor replacement ($150-$400, medium difficulty)
  • ●Exhaust leak repair ($200-$800, medium difficulty)
  • ●Addressing underlying engine issues like misfires ($300-$1500, variable difficulty)

Most Effective Fixes

Based on 40 owner reports. Success rates show how often each fix resolved the issue.

#1Spark Plug
67% success
9 reports
#2Torque Converter
50% success
4 reports
#3Catalytic Converter
33% success
9 reports
#4Ignition Coil
33% success
9 reports
#5Ignition Switch
33% success
9 reports

Start with the highest success rate fix when diagnosing P0420. If that doesn't resolve the issue, work down the list.

Root Causesfor P0420

Based on 57 owner reports

Confirmed Causes

  • 1
    bad exhaust manifold gasket i haven't got to yet(2% of cases)exhaust-manifold
  • 2
    likely the catalytic converter at fault and not worth replacing(2% of cases)catalytic-converter
  • 3
    them to be within acceptable voltages(2% of cases)

Common Misdiagnoses

What owners initially thought vs. what actually fixed the problem

  • sensorsi’m not fully sure(1 reports)

What Didn't Help

Repairs that owners tried but didn't fix the issue

come on all the timerunning hardcar nerdcar persondoing truck thingsenough for torque to flag as fail

Things to Check

Suggested areas to investigate based on similar cases

for any major issues before putting more money into2-3k of that towingand or possibly clean the o2 sensors or check the cat? ifbe the front manifold converter or rearcatalytic converter converter at fault and not worth replacingchevy truck

DIY Diagnostic Guide

Tools Needed

OBD ScannerMultimeter

DIY Diagnostic Checklist

Based on 57 owner reports

Progress0%

Tools Needed

OBD ScannerMultimeter
Step 1check: and or possibly clean the o2 sensors or check the cat? if so
Step 2check: engine light
Step 3check: engine light came on
Step 4check: engine light came on during the drive home
Step 5check: engine light came on yesterday
Step 6check: engine light is somewhat normal for me
Step 7check: engine light on
Step 8check: engine light to go out
Step 9check: engine still on with same code

Owner Statistics

Based on 1,106,821 real owner discussions • 30508 confirmed solutions

Dataset (1,106,821 records)
$2,149
Average Repair Cost
3%
Success Rate
20%
DIY Repairs
1,106,821
Data Sources

About This Data

Statistics are aggregated from real owner discussions on Reddit, automotive forums, and YouTube comments. All data is anonymized and represents actual repair experiences reported by vehicle owners.

Frequently Appears With

These codes often appear together in owner reports. Check these if you see P0420.

Based on analysis of 55 owner reports where multiple codes appeared together.

Issues by Vehicle

FORD

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2020 F-150

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2016 F-150

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2020 F-150

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2016 F-150

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2015 F-150

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2020 Mustang

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2020 Mustang

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2020 F-150

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2015 F-150

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2016 F-150

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2015 F-150

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2016 F-150

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2015 F-150

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2020 F-150

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2015 F-150

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2016 F-150

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2015 F-150

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2016 F-150

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2015 F-150

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2015 F-150

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2016 F-150

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100 sources

HONDA

2010 CR-V

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100 sources
2025 Civic

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2010 Civic

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89 sources
2010 CR-V

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100 sources
2025 CR-V

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100 sources
2025 CR-V

2025 Honda CR-V Oil Leak? How to Diagnose and Fix Wiring Issues

100 sources
2025 CR-V

2025 Honda CR-V Clicking Noise: What's Wrong and How to Fix It

100 sources
2010 CR-V

2010 Honda CR-V Oil Leak? Here's How to Check Your Injector Clips

100 sources
2010 CR-V

Why Is My 2010 Honda CR-V Engine Knocking?

100 sources
2010 CR-V

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100 sources
2010 CR-V

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100 sources
2025 CR-V

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100 sources
2020 Accord

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2010 CR-V

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100 sources
2020 Accord

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2020 Accord

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115 sources
2010 CR-V

2010 Honda CR-V Coolant Leak? Here's How to Fix It

100 sources
2020 Accord

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246 sources
2020 Fit

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116 sources
2020 Accord

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