Home › Diagnostic Codes › P0420
P0420 code: what it means, what causes it, and what it costs to fix
The scanner says P0420 and the check engine light is on. Here is what the code means, what usually causes it, and what the repair costs, so you do not pay for a catalytic converter you may not need.
P0420 means your car's computer thinks the catalytic converter is cleaning exhaust less effectively than it should. The full name is “Catalyst System Efficiency Below Threshold, Bank 1.” A worn-out converter is a common cause, but a failing oxygen sensor, an exhaust leak, or an engine misfire can set the same code. Published repair costs run from roughly $450 for sensor work to $1,000 to $4,000 or more for a converter, which is why the diagnosis matters more than the part.
What it means
The computer sees the converter on engine bank 1, the side with cylinder 1, doing a weaker cleanup job than its programming expects.
How urgent it is
A steady light with the car running normally can usually wait a few days. A flashing light means an active misfire, so stop driving and get help.
What it costs
Published figures range from about $450 for oxygen sensor work to over $4,000 for a converter, depending on the vehicle.
How the code is set
How your car decides it is P0420
Two oxygen sensors do the watching. One sits ahead of the catalytic converter (upstream) and one sits after it (downstream). The upstream sensor switches back and forth quickly as the engine adjusts fuel. A healthy converter smooths the exhaust out, so the downstream reading stays comparatively steady.
When the downstream signal starts to mirror the upstream one, the computer concludes the converter is not doing its job and stores P0420. “Bank 1” is the side of the engine that holds cylinder 1. Most inline four-cylinder engines have only one bank; horizontally opposed engines such as a Subaru boxer have two.
The code is a measurement, not a verdict. Repair guides from diagnostic-data companies such as Identifix stress that P0420 does not by itself prove the converter has failed.
Causes
What sets off a P0420 code
Listed roughly in the order a careful shop rules them out.
Engine misfire
Unburned fuel reaching the converter can overheat it. Look for P0300-series codes alongside P0420.
Exhaust leak
A leak near the downstream sensor lets outside air in and skews the readings the computer relies on.
Weak downstream sensor
A slow or failing oxygen sensor can report bad data. Replacing it costs far less than a converter.
Fuel mixture problems
Running too rich or too lean, such as the conditions behind P0171, strains the converter and can trigger the code.
Contaminated converter
Oil or coolant burning in the engine can coat the catalyst and reduce how well it works.
A worn-out converter
Age and mileage. Identifix notes converters can last 10 years or longer under normal conditions. This is the answer once everything else tests fine.
Symptoms
What P0420 feels like
Often nothing, apart from the check engine light. When a failing converter or its cause does show up on the road, drivers report a rough idle, less power, worse fuel economy, or a rotten-egg smell from the exhaust. A smell like that, or a car that feels down on power, is a reason to get the exhaust checked sooner rather than later.
Before you approve a repair
What a shop should check before replacing the converter
- Stored and pending codes, plus the freeze-frame data saved when the light came on
- Live fuel trim readings, both short-term and long-term
- Spark plugs, ignition coils and any active misfire
- The exhaust system for leaks, especially near the oxygen sensors
- How quickly the oxygen sensors respond
- Software updates and technical service bulletins for your vehicle
If a quote says you need a new converter and nothing in it says what was tested, ask what the shop measured. A good shop can show you the sensor readings or fuel trims behind the recommendation.
Cost
How much does it cost to fix P0420?
It depends on the cause, and the spread is wide. These are published figures from two repair-information sources, not quotes for your car.
| Repair | Published range | Source |
|---|---|---|
| Oxygen sensor replacement | About $454 to $550 | Identifix, citing RepairPal |
| Catalytic converter replacement | About $2,301 to $2,473 | Identifix, citing RepairPal |
| Converter replacement, wider range | About $1,000 to over $4,000 | ConsumerAffairs |
Your price will move with the make and model, the number of converters your car has, and whether the shop uses an original-equipment or aftermarket part. Lower figures you may see elsewhere, around $500 to $1,500, usually describe aftermarket or universal-fit converters on simpler vehicles. That spread is the main reason to compare written quotes. Two shops can look at the same code and quote very different jobs.
Check your warranty first
The federal emissions warranty covers a catalytic converter for 8 years or 80,000 miles, whichever comes first. Most other emissions parts are covered for 2 years or 24,000 miles. If your car is inside that window, the repair may cost you nothing at a dealer. California sets its own warranty periods.
Do not delete it
Removing the converter or swapping in a straight pipe on a road vehicle violates federal emissions law, and it is not a way to clear the code. Fix the cause instead.
Straight answers
P0420 questions drivers ask
Can I keep driving with a P0420 code?
Usually yes for the short term. P0420 on its own rarely stops a car from running. The risk is the cause behind it: if an engine misfire is setting the code, unburned fuel can overheat and damage the converter, so a small repair turns into a large one. If the check engine light is flashing, stop driving and have the car looked at.
Will P0420 go away on its own?
Sometimes the light turns off after several clean drive cycles, but if the cause is still there the code comes back. Clearing it with a scanner only erases the record. It does not fix anything.
Is it the catalytic converter or the oxygen sensor?
You cannot tell from the code alone. A shop compares upstream and downstream sensor readings and checks fuel trims, ignition and the exhaust for leaks first. If the sensors and engine test fine and the downstream signal still mirrors the upstream one, the converter is the likely culprit.
Can a catalytic converter cleaner or fuel additive fix P0420?
Additives are sold as a cheap fix, but they cannot rebuild a converter whose catalyst coating has worn out, and they do nothing if the real cause is a sensor, a leak or a misfire. Have the cause confirmed before spending money on either.
Why did P0420 come back after I replaced the converter?
The original cause may still be there, such as a misfire, an exhaust leak or a bad sensor, and it can damage the new converter too. A replacement part that is not matched to your vehicle can also set the code again. Ask the shop what it tested before the swap.
Will P0420 fail an inspection?
Florida does not require routine emissions testing for most passenger cars, and its program ended in 2000. States that do test will fail a car with an active catalyst code. Check your state’s rules if you plan to move or register a car elsewhere.
Is my catalytic converter covered by warranty?
Possibly. Under the federal emissions warranty, a catalytic converter is covered for 8 years or 80,000 miles, whichever comes first. Most other emissions parts are covered for 2 years or 24,000 miles. California has its own rules. Check your warranty booklet or ask a dealer before you pay.
Sources: Identifix, P0420 diagnosis guide; ConsumerAffairs, catalytic converter warranty; Pep Boys, Florida inspection rules. Costs and warranty terms vary by vehicle and state, so check your own paperwork.
Get quotes before you replace anything
Send one service request with your code and let local autoshops reply with pricing and timing.