P0420: Catalyst System Efficiency Below Threshold (Bank 1)
Moderate SeveritySafe to drive (short-term)Emissions
P0420 means your catalytic converter isn't cleaning exhaust gases efficiently enough. The downstream O2 sensor is detecting that converter performance has dropped below the acceptable threshold set by your vehicle's ECU.
What does P0420 mean?
P0420 means your catalytic converter isn't cleaning exhaust gases efficiently enough. The downstream O2 sensor is detecting that converter performance has dropped below the acceptable threshold set by your vehicle's ECU.
P0420 is a emissions-related diagnostic trouble code classified as moderate severity. When your vehicle's ECU detects this condition, it stores P0420 and illuminates the check engine light.
What are the symptoms of P0420?
If your vehicle has triggered P0420, you may notice one or more of these symptoms:
Check engine light illuminated
Slight decrease in fuel economy
Possible sulfur or rotten egg smell from exhaust
May fail emissions test
Reduced engine performance in some cases
What causes P0420?
Here are the most common causes of P0420, ranked by how likely they are to be the culprit:
mediumExhaust leak before or after catalytic converter
lowEngine misfire damaging the converter over time
lowIncorrect fuel type or engine oil burning
Is it safe to drive with P0420?
Generally yes, for short-term driving. Generally safe for short-term driving. Extended driving may damage the catalytic converter further and will fail emissions testing.
How do you diagnose P0420?
Follow these steps to pinpoint the root cause of P0420:
Read freeze frame data to identify conditions when code was set
Check for other codes (misfires, O2 sensor codes) that may be the root cause
Inspect exhaust system for leaks before and after the catalytic converter
Monitor O2 sensor waveforms with live data. Downstream sensor should show steady voltage if converter is working
Compare upstream vs downstream O2 sensor readings. If nearly identical, the converter is failing
How much does P0420 cost to fix?
Repair costs for P0420 vary depending on the root cause and your vehicle. Here are typical estimates:
Repair
Parts Cost
Labor Cost
Total Estimate
DIY Difficulty
Replace catalytic converter
€300–€1500
€100–€300
€400–€1800
Professional
Replace downstream O2 sensor
€50–€150
€60–€120
€110–€270
Moderate
Fix exhaust leak
€20–€100
€80–€200
€100–€300
Moderate
Prices estimated as of March 2026. Costs vary by region, vehicle, and shop.
P0420 on specific vehicles
P0420 behaves differently depending on your vehicle. Select your car below for model-specific causes, known issues, and adjusted cost estimates:
P0420 on Toyota Camry
Toyota Camry • 2007–2024
The 2007–2011 Camry with the 2.4L 2AZ-FE engine is especially prone to P0420 due to a known catalytic converter design issue. Toyota issued TSB 0111-09 addressing premature converter failure. The 2AZ-FE is also known for oil consumption, which contaminates the converter substrate over time.
What causes P0420 on a Toyota Camry?
Toyota CamryKnown catalytic converter design flaw on 2007–2011 2.4L models (TSB 0111-09)
Toyota CamryOil consumption in 2AZ-FE engines contaminates converter
P0420 on Honda Civic
Honda Civic • 2006–2024
On 2006–2011 Civics with the R18A engine, P0420 is frequently caused by a failing rear oxygen sensor rather than the converter itself. Always test the downstream O2 sensor first before committing to a converter replacement. Exhaust manifold cracking is another common issue on earlier models.
What causes P0420 on a Honda Civic?
Honda CivicRear O2 sensor degradation common on R18A engines
Honda CivicExhaust manifold cracking on 2006–2009 models
P0420 repair cost for Honda Civic
Estimated costs for Honda Civic ownersadjusted lower than average due to Honda-specific parts pricing
Repair
Parts Cost
Labor Cost
Total Estimate
DIY Difficulty
Replace catalytic converter
€270–€1350
€90–€270
€360–€1620
Professional
Replace downstream O2 sensor
€45–€135
€54–€108
€99–€243
Moderate
Fix exhaust leak
€18–€90
€72–€180
€90–€270
Moderate
Prices estimated as of March 2026. Costs vary by region, vehicle, and shop.
P0420 on Volkswagen Golf
Volkswagen Golf • 2005–2024
The Golf TDI (diesel) models don't typically trigger P0420. This code is predominantly found on TSI/TFSI petrol models. On the EA888 engine (2012+), check for oil consumption issues that can contaminate the converter. Turbo oil seal leaks can coat the converter substrate, causing premature failure.
Volkswagen GolfTurbo oil seal leaks coating converter substrate
P0420 repair cost for Volkswagen Golf
Estimated costs for Volkswagen Golf ownersadjusted higher than average due to Volkswagen-specific parts pricing
Repair
Parts Cost
Labor Cost
Total Estimate
DIY Difficulty
Replace catalytic converter
€345–€1725
€115–€345
€460–€2070
Professional
Replace downstream O2 sensor
€57–€173
€69–€138
€126–€311
Moderate
Fix exhaust leak
€23–€115
€92–€230
€115–€345
Moderate
Prices estimated as of March 2026. Costs vary by region, vehicle, and shop.
P0420 on BMW 3 Series
BMW 3 Series • 2006–2024
BMW 3 Series with N52/N55 engines commonly trigger P0420 alongside VANOS-related codes. The catalytic converter is integrated into the exhaust manifold on many models, making replacement significantly more expensive than standalone converters. Always check for VANOS issues causing rich running that degrades the converter.
What causes P0420 on a BMW 3 Series?
BMW 3 SeriesIntegrated manifold/converter design increases replacement cost
BMW 3 SeriesVANOS issues causing rich running that degrades converter
P0420 repair cost for BMW 3 Series
Estimated costs for BMW 3 Series ownersadjusted higher than average due to BMW-specific parts pricing
Repair
Parts Cost
Labor Cost
Total Estimate
DIY Difficulty
Replace catalytic converter
€420–€2100
€140–€420
€560–€2520
Professional
Replace downstream O2 sensor
€70–€210
€84–€168
€154–€378
Moderate
Fix exhaust leak
€28–€140
€112–€280
€140–€420
Moderate
Prices estimated as of March 2026. Costs vary by region, vehicle, and shop.
Related diagnostic codes
These codes are often seen alongside P0420 or indicate related issues:
Yes, if the underlying issue was temporary (like a tank of bad fuel), the ECU will clear it after several successful drive cycles. If it persists after 3–4 drive cycles, the converter or sensor likely needs attention.
Will P0420 cause my car to fail emissions?
Yes. P0420 directly relates to catalyst efficiency, which is a primary emissions test parameter. You'll need to resolve this code before passing an emissions inspection.
Can I just clear the code and ignore it?
You can clear it, but it will return if the root cause isn't fixed. Ignoring it long-term can lead to further converter damage and reduced fuel economy.
Is it the catalytic converter or the O2 sensor?
About 60–70% of P0420 cases are the converter itself, but the downstream O2 sensor is a common misdiagnosis. Always test the O2 sensor response before replacing the converter, as it's much cheaper.
