P0420
Catalyst System Efficiency Below Threshold (Bank 1)
Quick answer: P0420 means catalyst System Efficiency Below Threshold (Bank 1). Fixing the underlying cause typically runs $330 to $3,250 at an independent shop. Drive gently, service soon.
- Severity
- Moderate
- Safe to drive?
- Drive gently, service soon
- Typical repair
- $330 to $3,250
- Most likely cause
- A genuinely worn-out catalytic converter, common past 100,000 miles (the most likely cause and the expensive one)
What P0420 means
Your car has two oxygen sensors around the catalytic converter. When their readings start to look alike, the computer concludes the converter is no longer scrubbing exhaust the way it should, and it sets P0420. The catch is that a lazy downstream sensor or a small exhaust leak can fake this code, so P0420 is a conclusion about the converter, not proof of it.
Common symptoms
What drivers usually notice with this code.
- Steady check-engine light and often no drivability change at all
- Failed state emissions or smog test
- A rotten-egg or sulfur smell from the exhaust
- Mildly reduced fuel economy or slightly sluggish acceleration
- Rattling from under the car if the converter's internals have broken apart
Common causes
Ranked from most to least likely.
- 1A genuinely worn-out catalytic converter, common past 100,000 miles (the most likely cause and the expensive one)
- 2A failing downstream (post-cat) oxygen sensor reporting bad data
- 3An exhaust leak at a gasket, flex pipe, or weld near the converter, letting fresh air skew the readings
- 4An unresolved misfire or a rich condition that cooked the converter (fix that first, or the new converter dies too)
- 5An engine burning oil or coolant, which coats and poisons the catalyst
- 6A cheap aftermarket converter installed previously that never met spec
Can I keep driving?
Yes, this one is safe to drive with for a while, but it will fail an emissions test and the underlying cause can get more expensive the longer you wait.
Severity: Moderate. A flashing check-engine light always means stop, whatever the code says.
Ballpark cost
What P0420 usually costs to fix
A range, not a quote. What you pay depends on your vehicle, your area, and which of the causes above turns out to be the real one.
Typical parts and labor at an independent shop. Dealer rates run higher.
That spread is wide on purpose. P0420 points at a system, not a single part, so the real answer depends on what the diagnosis finds. Use the range to know what to expect. If an estimate comes in higher, it usually means the specific repair is more involved, and it is always fair to ask what is included.
Next steps
What to do next
The spread here is wide, from a roughly $330 oxygen sensor to a catalytic-converter job that can run past $3,000, so a careful diagnosis really matters. A good shop will show you the live data from both oxygen sensors and confirm there are no exhaust leaks and no stored misfire codes before recommending a converter. If a converter really is required, ask about a quality aftermarket unit versus the dealer part, and confirm whether your emissions warranty (often 8 years or 80,000 miles federally) still covers it.
Walk in informed
Ask for the diagnosis in writing and which test confirmed it, so you understand exactly what is being fixed. Fixxr shows you what the job typically costs near you, so you can approve the work with confidence.
Sources & further reading
- NHTSA — Check for vehicle safety recalls — look up open recalls for your vehicle by VIN, free
- FTC — Auto repair basics — your consumer rights at the repair shop
- AAA — Car repair and maintenance costs — independent guidance on what repairs should cost
Cost ranges and guidance reviewed July 2026.
Keep exploring
What the repair costs
P0420 FAQ
P0420 questions, answered
Can I drive with a P0420 code?
Yes, this one is safe to drive with for a while, but it will fail an emissions test and the underlying cause can get more expensive the longer you wait.What does the P0420 code mean?
Your car has two oxygen sensors around the catalytic converter. When their readings start to look alike, the computer concludes the converter is no longer scrubbing exhaust the way it should, and it sets P0420. The catch is that a lazy downstream sensor or a small exhaust leak can fake this code, so P0420 is a conclusion about the converter, not proof of it.What are the most common causes of a P0420 code?
The most likely causes, roughly in order, are a genuinely worn-out catalytic converter, common past 100,000 miles (the most likely cause and the expensive one); a failing downstream (post-cat) oxygen sensor reporting bad data; an exhaust leak at a gasket, flex pipe, or weld near the converter, letting fresh air skew the readings; an unresolved misfire or a rich condition that cooked the converter (fix that first, or the new converter dies too). The spread here is wide, from a roughly $330 oxygen sensor to a catalytic-converter job that can run past $3,000, so a careful diagnosis really matters. A good shop will show you the live data from both oxygen sensors and confirm there are no exhaust leaks and no stored misfire codes before recommending a converter. If a converter really is required, ask about a quality aftermarket unit versus the dealer part, and confirm whether your emissions warranty (often 8 years or 80,000 miles federally) still covers it.How much does it cost to fix a P0420 code?
Fixing the underlying problem behind a P0420 code typically runs $330 to $3,250 at an independent shop, including parts and labor. The exact figure depends on your vehicle and which part turns out to be at fault.
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