P0420: Catalyst System Efficiency Below Threshold (Bank 1)
Key takeaways
- P0420 means the engine computer's converter test failed on bank 1; it does not by itself prove the converter is the bad part.
- If the car runs normally you can usually keep driving while you book a diagnosis, but a flashing light, shaking or a sudden loss of power changes that.
- Rule out an exhaust leak, a lazy oxygen sensor and any misfire or fuel trim code first; those cost $100 to $800 against $1,000 to $2,500 for a converter.
- The catalytic converter is covered by the federal emissions warranty for 8 years or 80,000 miles, so check the odometer before you pay.
- 365 owner complaints filed with NHTSA mention P0420, and 85 of them are for the Kia Soul.
What P0420 means
Your catalytic converter isn’t cleaning the exhaust as well as the engine computer expects it to. Scanners call it “Catalyst System Efficiency Below Threshold (Bank 1).” The computer judges the converter by comparing two oxygen sensors: the upstream one, ahead of the converter, swings rich and lean many times a second as the engine trims fuel, while the downstream one, behind it, should stay fairly steady because a healthy converter stores oxygen and smooths that signal out. When the downstream signal starts copying the upstream one, the computer decides the converter has lost its storage capacity and sets P0420.
Bank 1 is the side of the engine with cylinder 1. A four-cylinder has only bank 1; on a V6 or V8 the other side’s converter sets P0430. The key point: the code means the test failed, not that the converter is proven bad.
Is it safe to drive?
Yes, in most cases. If the car starts, idles and pulls normally, P0420 on its own won’t strand you, and you can book a diagnosis in the next few weeks. What you lose in the meantime is your emissions inspection: a lit check engine light fails OBD testing in the states that run it.
The answer changes if the light starts flashing, the engine shakes, or the car feels choked and gutless on hills. A flashing light is an active misfire that is overheating the converter right now; ease off and get off the road. A converter that has melted inside can block the exhaust enough to kill power, and then it’s a tow. A strong sulfur smell is another sign the converter is struggling, covered in car smells like rotten eggs.
Causes, ranked
| Cause | How you confirm it | Typical cost (US independent shop, 2026) |
|---|---|---|
| Exhaust leak ahead of or near the downstream sensor | Ticking or puffing at cold start, soot at a joint, smoke test of the exhaust | $150–$600 |
| Aging oxygen sensor (usually the downstream one) | Slow or odd readings in live data on a warm engine; heater codes like P0141 | $150–$400 per sensor |
| Misfire, rich running or oil burning upstream | Other stored codes (P0300, fuel trim codes), high fuel trims, oil use between changes | $150–$800 for the usual fix (plugs, coil, injector) |
| Engine software needs an update | Shop checks for a bulletin on your make, model and engine | $0–$200 |
| Converter worn out | Downstream sensor mirrors upstream after the above are ruled out; rattle inside the shell | $1,000–$2,500 per converter |
On an older, high-mileage car where none of the cheaper causes turns up, the converter itself is the most common final answer. Trucks, some imports, and California-compliant converters sit at the top of that range or above it.
What to check first
- Read every stored code, not just P0420. Misfire or fuel trim codes get fixed first; a converter fed raw fuel or burning oil will fail again.
- Listen at a cold start for ticking or puffing near the exhaust manifold and flex pipe. Leaks often seal up as the metal warms.
- Look at the downstream sensor in live data on a fully warm engine at a steady cruise. A fairly flat signal means the converter is probably doing its job and the problem lies elsewhere; one that switches like the upstream sensor points at the converter. Our O2 sensor guide explains the readings.
- Ask about bulletins and warranty. Several makes treat P0420 as a known issue. Ford bulletins for some Expedition, Navigator and F-150 hybrid models say high-sulfur fuel may set P0420 and P0430, Subaru issued a diagnostic procedure for Outback and Ascent, and Honda has asked dealers for cars with 1.5L turbo engines showing the code. Separately, the converter carries a federal 8-year/80,000-mile emissions warranty.
- Clear it and drive only after a repair. Clearing wipes the readiness monitors, and the catalyst test can take several days of mixed driving to run again, so don’t clear it the week of an inspection.
Among owner complaints, 365 filed with NHTSA mention P0420, 85 of them for the Kia Soul, mostly 2012–2016 model years. That is a count of owner reports, not a confirmed defect.
What it costs
The cheap outcome is an exhaust leak or an oxygen sensor at $150–$600. The common outcome on a well-maintained car is a sensor or an upstream fix like plugs or a coil, up to about $800. The expensive outcome is a converter at $1,000–$2,500 each, more on trucks and in California. A diagnosis at $100–$180 is what keeps you from buying a $1,500 converter to fix a $200 leak.
Related codes
P0420 often shows up with P0430 on two-bank engines, with oxygen sensor codes like P0138, or with misfire codes that caused the damage. P0421 is a closely related warm-up catalyst efficiency code used by some makers.
What owners report to NHTSA
365 complaints filed with NHTSA mention P0420, 307 of them since 2015, across 110 different models. The component owners most often filed it under: engine.
Most-reported models
| Vehicle | Complaints | Model years reported |
|---|---|---|
| KIA SOUL | 85 | 2012, 2013, 2014, 2015, 2016 |
| NISSAN ALTIMA | 19 | 2002, 2003, 2005 |
| FORD FOCUS | 15 | 2012, 2013, 2014, 2015, 2016 |
| HONDA ODYSSEY | 12 | 1999, 2000, 2005, 2018, 2019 |
| HYUNDAI TUCSON | 12 | 2005, 2016, 2017, 2019, 2020 |
| HONDA ACCORD | 10 | 1998, 1999, 2003, 2008, 2010 |
| HONDA PILOT | 10 | 2016, 2017, 2019 |
| CHEVROLET EQUINOX | 9 | 2010, 2011, 2012, 2014, 2015 |
| CHEVROLET TRAVERSE | 9 | 2010, 2018, 2019, 2020, 2021 |
| FORD F-150 | 7 | 2006, 2018, 2019 |
Manufacturer service bulletins that name P0420
282 technical service bulletins and manufacturer communications filed with NHTSA mention P0420, from 28 makes. A bulletin means the manufacturer has documented a known cause or a fix procedure for that model. Examples:
- VOLVO VOLVO TRUCK, bulletin 10203159: Volvo Chassis - Diagnostic Trouble Code ( DTC ) P0420 Illuminating The Malfunction Indicator Lamp (MIL)
- VOLVO CROSS COUNTRY (2001, 1994, 1995, 1996), bulletin 10209840: Volvo Chassis - Diagnostic Trouble Code ( DTC ) P0420 Illuminating The Malfunction Indicator Lamp (MIL)
- HONDA FIT (2017, 2023, 2024, 2021), bulletin 11010549: Dealer Message - American Honda Motor Co., Inc. (AHM) is searching for certain 2018-2024 Accord 1.5Ls, 2021-2024 Civics 1.5Ls, 2023-2024 CR-Vs 1.5Ls, and 2017 Fits with customer complaint of Check Engine Light (CEL) on w…
- HONDA ACCORD (2018, 2019, 2020, 2021), bulletin 11011792: Dealer Message - American Honda Motor Co., Inc. (AHM) is searching for certain 2018-2024 Accord 1.5Ls, 2021-2024 Civics 1.5Ls, 2023-2024 CR-Vs 1.5Ls, and 2017 Fits with customer complaint of Check Engine Light (CEL) on w…
- FORD BRONCO SPORT (2021, 2022, 2023, 2020), bulletin 10241542: Some 2020-2022 Escape built and 2021-2023 Bronco Sport vehicles, built on or before 28-Sep-2022, equipped with a 1.5L Ecoboost engine and a 8F35 transmission may exhibit an illuminated malfunction indicator lamp (MIL) wi…
- FORD F-150 (2022, 2023), bulletin 10244800: Some 2022-2023 F-150 hybrid vehicles equipped with a 3.5L PowerBoost engine may exhibit an illuminated MIL with diagnostic trouble codes (DTCs) P0420 and/or P0430 stored in the powertrain control module (PCM). This may b…
Sources: NHTSA Office of Defects Investigation complaint database and manufacturer communications file, snapshots September 2026. A complaint or bulletin that mentions a code is a report about that vehicle, not a statement that the code has one cause.
Frequently asked questions
Can I drive with a P0420 code?
Usually yes. If the engine idles smoothly, pulls normally and the light is steady, P0420 is an efficiency warning, not a breakdown warning. Get it diagnosed within a few weeks, and sooner if you have an emissions inspection coming. Stop driving if the light starts flashing or the car suddenly loses power.
Will a new oxygen sensor fix P0420?
Sometimes. An aging downstream sensor can make a good converter look bad, but so can an exhaust leak, and a worn-out converter will set the code again with a brand-new sensor. Have the shop check the sensor's live data before replacing it, and fix any misfire or fuel trim codes first.
Do catalytic converter cleaners fix P0420?
Rarely for long. A bottle of cleaner can sometimes make a borderline converter pass for a while, but it can't restore a converter that has lost its coating or melted inside. It's cheap enough that some owners try it; don't count on it, and don't let it delay finding a misfire or oil-burning problem.
Is P0420 covered by warranty?
The converter often is. Federal rules require carmakers to cover the catalytic converter for 8 years or 80,000 miles, and California-certified cars get longer coverage on some parts. The repair has to go through a manufacturer-authorized shop, so call the dealer first if your car is inside that window.
What is the difference between P0420 and P0430?
Only the side of the engine. P0420 is bank 1, the side with cylinder 1. P0430 is the same converter test on bank 2, which only exists on V6, V8 and other two-bank engines. If both set together, look for something that affects the whole engine, like a misfire, fuel problem or oil burning.
References
- NHTSA Office of Defects Investigation — vehicle owner complaint flat files
- NHTSA Office of Defects Investigation — manufacturer communications (service bulletin) flat files
- Cal. Code Regs. tit. 13, § 1968.2 — OBD II requirements, catalyst monitoring (Cornell LII)
- EPA — Frequent questions related to transportation and air (federal emissions warranty)