The P0420 code means your engine computer has decided the catalytic converter on bank 1 is not cleaning the exhaust as well as it should. Its official description is “catalyst system efficiency below threshold bank 1”. Carmakers have written a lot about this code for their own dealers, and what they say should change the order in which you fix it.
What the P0420 code means
Your engine has one oxygen sensor in front of the catalytic converter and another behind it. A working converter stores oxygen and evens out the exhaust, so the rear sensor’s signal stays fairly flat while the front one swings back and forth. When the rear signal starts to follow the front one, the computer concludes the converter can no longer store oxygen and sets P0420.
The computer never measures the converter itself. It only compares two sensors, so anything that changes those readings can set the code on a converter that still works: a lazy sensor, an exhaust leak, a misfire, or the computer’s own software.
“Bank 1” is the side of the engine with cylinder number 1. On a V6 or V8, the matching fault on the other side (bank 2) is P0430, and many bulletins mention the two together.
What causes P0420, according to manufacturers’ service bulletins
Carmakers publish technical service bulletins (TSBs) to tell dealers how to fix known problems. NHTSA holds 298 of them that mention P0420. Counting each revision once and leaving out non-repair notices, 210 distinct repair bulletins remain.
What manufacturers prescribe for P0420
Share of 210 distinct repair bulletins that mention P0420, by the fix they call for
Source: NHTSA manufacturer communications (technical service bulletins), received 2010–2026. Revisions of the same bulletin are counted once. A bulletin can call for more than one fix.
A software update for the engine computer is the most common fix by a wide margin. Misfire, ignition and fuel injector repairs come second, since unburned fuel overheats the converter and throws off the sensors. A new catalytic converter is the prescribed fix in only a handful of bulletins.
A shop that replaces the converter first may clear the light for a while. If the real cause was a misfire or outdated software, the code comes back on the new part.
The bulletins below cover the same code on very different vehicles:
| Bulletin | Vehicles | What it says |
|---|---|---|
| TSB 22-2260 Ford · Jan 2023 | Expedition 2020–2021 F-150 2018–2020 | Ford traces P0420 and P0430 on these trucks to high sulfur content in the fuel. The dealer runs a de-sulfation procedure on the converters. |
| TSB 23-2337 Ford · Oct 2023 | F-150 2022–2023 | Same sulfur explanation for the 3.5L PowerBoost hybrid, with a procedure that cleans sulfur out of the converters. |
| A20-100 Honda · Feb 2021 | Odyssey 2018 Pilot 2016–2017 Ridgeline 2017 | The engine computer can read normal sensor signals as a worn converter. Debris inside the injectors can add misfire codes. Software update plus injector checks. |
| A22-045 Honda · Oct 2022 | Civic Hybrid 2014–2015 | Sensors may judge a healthy catalyst as deteriorated. Honda updates the fuel injection software when P0420 is stored. |
| SC176Y_TSB Kia · Oct 2024 | Soul 2012–2016 | Applies to Souls that already had recall SC176. Covers diagnosis of catalyst deterioration, converter replacement and engine inspection or replacement. |
| 24M01 S1 Ford · Aug 2024 | Explorer 2020 Explorer Police Intercept 2020 | Converters on the 3.3L can crack or suffer heat damage. Failed converters are replaced and the wiring harness is inspected. |
| N232427150 Chevrolet · Dec 2023 | 6500HD 2019–2021 Low Cab Forward 6500 2018 | On these diesel trucks a failed DPF fuel injector can set P0420. Covered at no charge under special coverage. |
| 20-FL-010H Hyundai · Oct 2020 | Tucson 2019 | The repair pairs an engine computer update with a new exhaust manifold catalyst. |
| 09-110-23R Subaru · Nov 2023 | Ascent 2019–2021 Crosstrek 2017–2023 Forester 2017–2023 Impreza 2017–2023 +3 more models | The diagnostic procedure Subaru dealers follow when P0420 sets, across most of the 2017–2023 lineup. |
| PIP6067 Cadillac · May 2025 | XT4 2024–2025 | Dealers replace the converter together with both oxygen sensors as one assembly while engineering studies the cause. |
P0420 symptoms owners report
The code itself does not change how the car drives. Owners notice the warning light and whatever problem caused the code. Across 367 owner complaints to NHTSA that mention P0420, these are the things people describe.
What owners describe alongside P0420
Share of 367 NHTSA owner complaints that mention P0420
Counted by phrases in the complaint text. One complaint can mention several things.
Two out of three complaints mention the check engine light, and roughly three in ten owners say the converter had already been replaced. Loss of power and stalling appear less often. When those come with P0420, look for a misfire or a clogged converter first. Owners who gave their mileage were at a median of 102,259 miles when the problem appeared.
Which vehicles report P0420 the most
P0420 by make
Owner complaints and service bulletins that mention P0420, top 10 makes
Owner complaintsService bulletins
Bulletins are counted per make and bulletin number; one bulletin can cover several models and years.
Kia has the most owner complaints, and almost all of them come from one model. The Kia Soul alone accounts for about a quarter of every P0420 complaint in the database, which matches Kia’s bulletin on catalyst problems in 2012–2016 Souls listed above. In bulletins, Ram leads, mostly with engine computer updates spread over several model years.
| Make | Model | Complaints |
|---|---|---|
| Kia | Soul | 92 |
| Ford | Focus | 18 |
| Nissan | Altima | 16 |
| Hyundai | Tucson | 12 |
| Honda | Odyssey | 10 |
| Honda | Pilot | 10 |
| Chevrolet | Equinox | 9 |
| Chevrolet | Traverse | 9 |
| Ford | F-150 | 8 |
| Dodge | Journey | 7 |
P0420 complaints by model year
Model years 2005–2023; peak year 2016 highlighted
Complaints rise through the early 2010s and peak with the 2016 model year, then fall off for newer vehicles.
Is it safe to drive with a P0420 code?
For most cars, yes, for a short time. P0420 alone does not stop the engine or put it into limp mode. Weeks of driving with it is another matter. In states with OBD-based testing the car will fail inspection, a second fault can set its own code behind a light that is already on, and the cause keeps wearing the converter.
Stop driving and have it checked if:
the check engine light is flashing, which means an active misfire that can overheat the converter. The same applies when the engine misfires or loses power, when you smell sulfur or something burning, or when the converter area glows or runs unusually hot.
How to fix a P0420 code, in the right order
Going by the bulletin data, start with the cheap and likely causes and leave the converter for last.
- Ask a dealer to run your VIN for open bulletins and recalls. A software update may be all the car needs. Federal law also covers the catalytic converter for 8 years or 80,000 miles under the emissions warranty.
- Repair any misfire first. Misfire codes (P0300 to P0308), fuel trim codes and air-fuel sensor codes often sit behind P0420, and fixing them can make it go away.
- Look for exhaust leaks in front of the rear oxygen sensor. Outside air pulled in through a leak makes the converter look worse than it is.
- Watch both oxygen sensors in live data. A slow or contaminated rear sensor can set P0420 on a converter that works.
- Think about fuel. Ford blames high-sulfur fuel for P0420 on some trucks, and a tank from a different station followed by a long highway drive gives the readiness monitor a fair chance to run again.
- Replace the converter once everything above checks out. Buy an OEM or emissions-certified unit. California only allows aftermarket converters approved by its Air Resources Board, so check your own state’s rules before ordering.
You will also see oxygen sensor “spacers”, converter removal and tunes that switch off the rear sensor sold as P0420 fixes. All three count as emissions tampering under the federal Clean Air Act. The car fails inspection with them, and the original fault is still there.
P0420 vs P0430
Both codes describe the same test failing, P0420 on bank 1 and P0430 on the opposite side. When both appear together, suspect something the two banks share, such as fuel quality or engine software. Two converters wearing out on the same day is the less likely explanation.
P0420 questions owners ask
How serious is a P0420 code?
On its own it is a moderate problem. The car usually runs normally, but it will fail an OBD-based emissions inspection, and whatever is wearing the converter keeps doing so. Get it checked the same day if the check engine light starts flashing or the engine loses power.
What is the most common cause of P0420?
In manufacturer service bulletins the most common fix is a software update to the engine computer. Misfire and fuel injector repairs come second. Only one bulletin in twenty calls for a new converter. On older, high-mileage cars a converter that has simply worn out is more common, and misfires or oil burning can wear it out early.
How much does it cost to fix P0420?
A software update or a converter still under the federal emissions warranty (8 years or 80,000 miles) costs you nothing. A new converter plus labor is the expensive end. Rule out bulletins and misfire codes before paying for one.
Is it safe to drive with P0420?
Usually yes for short trips if the engine runs smoothly and the check engine light stays on without flashing. Stop and get it checked if the light flashes or the engine misfires, because unburned fuel can overheat the converter. A sulfur smell or a glowing converter also means stop.
Can a bad oxygen sensor cause P0420?
Yes. The code comes from comparing the oxygen sensor before the converter with the one after it, so a slow or contaminated sensor can make a good converter look bad. Oxygen or air-fuel sensors come up in about one in ten NHTSA owner complaints about P0420.
Will a P0420 code clear itself?
It can. If the cause was temporary, such as high-sulfur fuel, the computer may pass the catalyst test again after several drive cycles and turn the light off. If the converter or a sensor is really failing, the code comes back. Clearing it with a scanner also resets the readiness monitors, and an emissions test will notice that.
Is P0420 bank 1 or bank 2?
P0420 is bank 1, the side of the engine with cylinder 1. The same fault on bank 2 is P0430. Most inline four-cylinder engines only have bank 1, so they set P0420 and never P0430.