Catalyst System Efficiency Below Threshold (Bank 1)
The P0420 code means the vehicle's onboard diagnostic system has detected: Catalyst System Efficiency Below Threshold (Bank 1). This is a moderate severity code.
- Keep driving?
- Yes, but fix soon
- DIY difficulty
- medium
- Estimated cost
- $95-$250 for O2 sensor; $350-$2,200 for catalytic converter replacement
Common Misconceptions / Fact Check
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Commonly claimedP0420 means you automatically need a new catalytic converter.
VerifiedNot necessarily. P0420 reports low catalyst efficiency, but a worn upstream/downstream O2 sensor, an exhaust leak, or a fuel-trim problem can set it. Rule those out first -- replacing the converter without fixing the real cause often brings the code back.
Reads enhanced codes and shows real-time O2 sensor data. Verify the cat is actually bad before dropping $1,000+ on a replacement.
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Common Symptoms
- Check Engine Light on (steady)
- Slight reduction in fuel economy
- Sulfur or rotten-egg smell from exhaust in some cases
- May fail emissions testing
Probable Causes (Ranked by Likelihood)
- Worn or failed catalytic converter Very High
- Faulty downstream (post-cat) oxygen sensor High
- Engine oil consumption fouling the cat Moderate
- Coolant leak into combustion chamber Moderate
- Rich-running condition burning the catalyst Moderate
- Exhaust leaks before or after the cat Low
- High-sulfur or contaminated fuel (can trip the monitor without a failed cat -- some makers have issued TSBs on this) Low
Step-by-Step Diagnostic Procedure
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Check freeze frame data: if the code set under steady highway conditions, the cat is more likely the culprit. Under mixed driving, suspect a sensor.
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Compare upstream (pre-cat) and downstream (post-cat) O2 sensor waveforms with a scan tool. Downstream should show a slow, lazy signal (0.1-0.9V with few switches). If it mirrors upstream switching, the cat is not storing oxygen properly.
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Inspect for exhaust leaks upstream of the downstream sensor: a leak can pull in fresh air and falsely indicate a bad cat.
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Check for oil fouling: blue smoke, excessive oil consumption, or a milky oily residue inside the tailpipe.
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Perform a temperature test with the engine warm and held at 2,000-2,500 RPM (not at idle): the cat outlet should run about 50-100 degrees F hotter than the inlet. Little or no rise -- or an outlet cooler than the inlet -- indicates a dead converter. At idle a healthy cat can show almost no difference, so do not condemn it from an idle reading.
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If the downstream O2 sensor is original and over 100,000 miles old, replace it before replacing the cat: a sluggish sensor mimics a failed catalyst.
Common Fixes by Vehicle
What techs usually find when diagnosing P0420 on specific vehicles — tap your vehicle for the fix and the exact part:
2007-2017 Toyota Camry/Corolla 2.5L moderate DIY
Toyota cats last longer than most but fail around 150K miles. Before replacing, check for exhaust leaks and test the downstream O2 sensor. If the cat is truly failed, aftermarket cats from Walker or Eastern Catalytic save 60% vs dealer. CARB-compliant cat required in California.
Labor: 1-2 hours2008-2019 Honda Accord/Civic 2.4L easy DIY
Often a downstream O2 sensor (sensor 2) rather than the cat itself. Test with a scanner that shows live O2 data -- if downstream sensor voltage is steady at 0.45V, the sensor is lazy. Replace sensor before cat. Use Denso 234-4350 for exact OE fit.
Labor: 30 min2004-2018 Nissan Altima/Sentra 2.5L QR25DE easy (PCV) DIY
Nissan has a known issue with premature cat failure. TSB NTB15-094 covers this. Often related to oil consumption on QR25DE engine contaminating the cat. Fix oil consumption first (PCV valve, piston rings) or the new cat will fail too. Check for blue smoke on startup as a telltale sign.
Labor: 15 min (PCV), 2 hours (cat)2009-2020 Subaru Outback/Forester 2.5L easy (gasket) DIY
Subaru boxer engines run catalytic converters very close to the engine. Heat cycling causes premature failure around 100-120K. Check for exhaust leaks at the header-to-cat gasket first (cheap fix). Subaru extended warranty applies on some 2012-2016 models -- check with dealer before paying out of pocket.
Labor: 30 min (gasket), 2 hours (cat)Frequently Asked Questions
Will P0420 cause my car to fail emissions?
Yes. In most OBD-II emissions states, any stored fault code with the MIL illuminated results in an automatic failure, regardless of tailpipe emissions results.
Can a bad O2 sensor cause P0420?
Yes: a slow or lazy downstream oxygen sensor mimics the behavior of a failed catalytic converter. Always test or replace the downstream O2 sensor before purchasing a catalytic converter.
Does using premium gasoline fix P0420?
No. Fuel grade does not affect catalytic converter efficiency. Additives marketed as 'cat cleaners' have no proven effect on a mechanically failed converter.
Could a software update fix P0420 instead of a new catalytic converter?
Sometimes. Several manufacturers have issued technical service bulletins (TSBs) that reflash the PCM with a revised catalyst-monitor calibration. If the converter is actually within spec, the update can stop a false P0420. Check for an applicable TSB for your year and engine before buying a converter.
Sources
This page is built from documented references. Verify against your own service info before repair work.
P0420 on Diesel, EV & Equipment
The same code ID appears across other engines and platforms. The diagnostic flow varies by manufacturer — these are the platform-specific breakdowns:
See All Codes & Fixes for Your Vehicle
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