P0420
Moderate severity · Repair $400–$2,200
Catalyst System Efficiency Below Threshold (Bank 1)
Your catalytic converter is not cleaning exhaust gases efficiently enough. Your car still drives, but it will fail emissions.
Reviewed by an ASE-Certified Technician · Updated May 2026 · Based on 84,291 verified scans
What P0420 actually means
Your car's computer is saying that the catalytic converter, which cleans your exhaust, isn't doing its job well enough. Think of it like a clogged filter for your exhaust pipe. The car will still drive fine, but it will fail an emissions test and should be fixed within a few weeks. The most common fix is either a $50–$120 sensor replacement or a $600–$2,000 converter replacement.
Top causes, ranked
68%
Failed catalytic converter
The converter substrate has broken down internally. Honeycomb structure physically degraded or poisoned by oil/coolant contamination.
Part: $200–$1,400Labor: $100–$300DIY: Medium
22%
Faulty downstream O2 sensor
The rear oxygen sensor is reading incorrectly. Always test sensors before replacing the converter, a $50 sensor can clear this code.
Part: $30–$120Labor: $50–$100DIY: Easy
10%
Exhaust leak before the converter
A manifold crack or gasket leak introduces extra oxygen into the stream, confusing the O2 sensors and triggering a false P0420.
Part: $20–$150Labor: $80–$250DIY: Hard
Is it safe to drive?
Short-term: yes. P0420 is not a safety hazard. Your car will run normally. But driving for weeks without repair risks damaging your O2 sensors, and the vehicle will fail emissions inspection. Plan to diagnose and repair within 2–4 weeks.
What it feels like while driving
Honest answer: usually nothing. That's what makes P0420 so easy to ignore, and how a $150 sensor fix becomes a $1,800 converter replacement.
Most common · 70%+
No symptoms at all
Car runs perfectly. The only sign is the check engine light. The converter is slowly losing efficiency, not failed yet.
Sometimes present
Slight power loss under load
A partially clogged cat restricts exhaust flow. Engine feels lazy merging onto the highway or climbing hills.
Advanced stage
Rotten egg smell + rough idle
Sulfur smell usually means the cat is damaged or the upstream O2 sensor is dumping raw fuel through. Repair is urgent.
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Most affected vehicles
| Vehicle | Frequency | Avg cost | Mileage |
|---|
| Toyota Camry (2002–2011) | Very high | $650 | 85k–140k |
| Toyota Corolla (2003–2013) | Very high | $600 | 90k–160k |
| Honda Accord (2003–2012) | High | $700 | 80k–130k |
| Chevy Silverado (2005–2015) | High | $950 | 75k–120k |
| Ford Escape (2008–2014) | Moderate | $850 | 70k–110k |
| Subaru Outback (2005–2012) | Moderate | $1,100 | 60k–100k |
Diagnosis steps
-
Scan the code and confirm it's only P0420
Use an OBD2 scanner. If P0300–P0308 misfires are also present, fix those first, they destroy new converters.
-
Pull freeze frame, check O2 sensor voltages
The downstream O2 sensor voltage at the moment the code set is the key data point. A reading that mirrors the upstream sensor (oscillating 0.1–0.9V) means likely failed cat. A flatlined reading near 0V or 1V means likely bad sensor. This one number changes your entire repair path.
-
Get steps 3+ for your exact vehicle
Run a personalized diagnosis for torque specs, connector pinouts, live-data targets, and OEM part numbers for your year/make/model.
P0420 vs P0430
Both codes flag catalytic converter efficiency, but on different exhaust banks.
| Attribute | P0420 | P0430 |
|---|
| Bank | Bank 1, driver side (or only bank on 4-cyl) | Bank 2, passenger side |
| Engines | All gas (4-cyl, V6, V8) | V6 and V8 only |
| Repair cost | $150–$2,400 | $150–$2,400 |
| Both at once? | Yes, GM 5.3L V8s commonly throw both together (oil consumption poisons both cats) |
Community reports
MarcusK, Pittsburgh · 2013 Toyota Camry · 112k mi
P0420 came on at 112k. One shop wanted $1,800 for a new cat. The other tested the O2 sensor first and found it reading flat. $65 sensor from RockAuto, 20-minute job. Code cleared. Always test the sensor first.
SarahR, Denver · 2007 Honda Accord · 98k mi
P0420 plus a faint burning smell. Cracked exhaust manifold gasket was leaking onto the cat. Replaced the gasket for $180 and reset the code. 1,000 miles later, no return. Saved me from a $1,200 converter.
JeffL, Orlando · 2009 Chevy Silverado 5.3L · 141k mi
Silverado burning oil from AFM lifter issues. Poisoned both cats. $2,800 total. If you have a GM 5.3L with P0420/P0430, check oil consumption first.
TinaM, Seattle · 2011 Subaru Outback · 88k mi
Dealer wanted $1,400 for an OEM cat. Ran AmpAuto diagnosis first, it flagged the downstream O2 sensor based on freeze frame. $78 sensor. Code hasn't returned in 4 months. Saved $1,300+.
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