P0420 means the engine control module ran its catalyst efficiency monitor on bank 1 and found the converter storing less oxygen than the calibration requires. P0430 is the same test on bank 2. Bank 1 is the cylinder bank containing cylinder 1; on an inline engine there is only one bank and one code. The module is not measuring emissions directly. It is measuring how much the downstream oxygen sensor's signal still swings compared with the upstream sensor's, and a converter that has lost its oxygen storage capacity lets the downstream signal follow the upstream one.
Severity: low for the engine, high for the wallet if diagnosed wrong. Can you keep driving? Yes. The engine runs normally with P0420 alone. The vehicle will fail an emissions inspection, and in states with inspection programs the check engine light itself is a rejection. The exceptions that need attention sooner: a flashing check engine light (a misfire, which is destroying the converter right now; see flashing versus steady), a rotten-egg smell under load, a rattle from the exhaust, or a loss of power at highway speed, which point at a converter that is physically breaking up or plugging.
This page is the long version. The short explanation of the monitor and the two-sensor comparison lives at P0420 catalyst efficiency explained; what follows is the diagnosis, the causes in the order they are found, and what the manufacturers have documented.
How the monitor decides
A three-way catalyst works by storing oxygen during lean moments and releasing it during rich moments, so that the reactions inside it see a steady, stoichiometric gas even though the engine's mixture oscillates several times a second. A fresh converter's oxygen storage smooths those oscillations almost completely: the upstream oxygen sensor swings rich-lean-rich-lean and the downstream sensor sits nearly flat. A worn converter has lost the ceria that does the storing, and the downstream sensor begins to swing along with the upstream one.
The module measures that. Under a defined set of conditions (engine warm, steady cruise in a speed and load window, closed-loop fueling, fuel trims within limits, no misfire, no other relevant codes), it either counts the downstream sensor's transitions against the upstream sensor's over a window, or on newer calibrations it deliberately forces the mixture rich and lean and times how long the converter takes to pass the change through. When the ratio or the delay crosses the calibrated threshold on two consecutive drive cycles, P0420 is stored and the light comes on.
Three consequences of that design drive everything below:
- The test needs the converter to be at operating temperature and the mixture to be right. Anything that makes the mixture wrong (a leak, a bad injector, a lazy upstream sensor) or the converter cool (short trips, a leaking exhaust before the sensors) can fail the test on a healthy converter.
- The test is only as good as the two sensors. A downstream sensor that has become fast, or an upstream sensor that has become slow, changes the ratio without any change in the converter. Comparing the two sensors' live waveforms is the core of the diagnosis.
- The threshold is a calibration. Several manufacturers have issued software that changes it, because the original setting flagged converters that were still within emissions limits. That is the recurring theme in the bulletins.
What the manufacturers documented
These are filed with the National Highway Traffic Safety Administration and are quoted by number so they can be checked.
Volkswagen Atlas 2018 to 2019, 3.6 L (technical bulletin 01-20-01, March 2020): P0420 or P0430. Volkswagen's instruction is blunt: do not diagnose or replace components for these codes before performing the hardware replacement and the update in this bulletin. The repair pairs an improved catalytic converter for long-term durability with engine software that reduces thermal stress on it. The software also carries changes for P0456 and the P2096 to P2099 fuel trim codes, which were moved to a separate bulletin. On these vehicles the converter was replaced, but under a bulletin with a calibration, not on the strength of the code alone.
Kia Optima 2011 to 2015, Sorento 2012 to 2015, Sportage 2014 to 2016 with the 2.4 L GDI (TSB ENG 160, October 2016): P0420, sometimes with P0171, occurring most commonly during prolonged engine braking down a steep grade. The instruction: first inspect the warm-up catalyst's function with the factory tool, then perform the ECU upgrade; replace the converter only if the inspection confirms it has deteriorated. The order is inspection, software, part.
Acura TLX 2015 to 2017 V6 (Service Bulletin 19-049, August 2019): P0420 on the rear bank. Probable cause in Honda's words: the fuel injection software may misinterpret sensor inputs as a deteriorated catalytic converter. Corrective action: update the software. No part.
Ford Ranger 2019 to 2020 (TSB 21-2021, March 2021): P0420 due to various software parameters within the PCM. The correction reprograms the PCM and replaces the converter under the emissions warranty, with the converter-to-turbo nuts and stabilizer bar hardware listed for the job.
Jeep Wrangler 2024, 2.0 L turbo, built on or before January 30, 2024 (TSB 18-011-25, March 2025): P0420 with P0017 (crank and cam timing misalignment), P000A (cam position slow response) and P219A (air-fuel imbalance), possibly with a no-crank. Cause: PCM software. The repair may replace the oil control valves and the converter and reprograms the PCM. The cam timing codes are the tell: a cam that responds slowly on this engine upsets the mixture the catalyst monitor depends on, and the valves that control the cam were the first item on the repair list.
Mitsubishi Lancer Evolution 2006 (service campaign SC-13-002, April 2013): P0420 after continuous over-redline operation raised catalyst temperatures enough to damage the converter. The campaign reprograms the PCM to induce the driver to upshift or lift, replaces the converter if damaged, and extended the converter warranty.
Kia Soul 2012 to 2016 with the 1.6 L GDI (subsequent repair action SC176Y, October 2024): on vehicles that had an earlier engine recall completed, catalyst efficiency deterioration or P0420, with or without a P030x misfire, is handled by inspecting and if necessary replacing both the converter and the engine sub-assembly. The converter failed because the engine was consuming oil or misfiring, and replacing only the converter would have repeated the failure.
Seven vehicles, and the converter alone was the fix on none of them. That is the frame for the rest of this page.
What actually kills a converter
A three-way catalyst does not wear out on its own within the life of most vehicles. It is killed by something upstream, and unless that something is found, a new converter dies the same way. The killers, in order of frequency:
Misfire. Unburned fuel and oxygen reach the converter together and burn inside it. Catalyst temperatures climb past the substrate's limit and the ceramic melts or crumbles. This is what the flashing check engine light warns about, and it is why P0420 with a history of P0300 through P0308 is an engine problem first. See P0300 misfire codes.
A rich mixture. A leaking injector, high fuel pressure, a stuck purge valve or a lazy upstream sensor feeding the converter more fuel than it can process. The converter runs hot and ages fast. P0172 or P0175 in the history is the tell; see P0175.
Oil or coolant in the exhaust. Oil consumption from worn rings or valve seals coats the catalyst with phosphorus and zinc from the oil's additives; coolant from a head gasket coats it with silicates. Neither burns off. The Kia Soul action above exists because an engine that burns oil takes its converter with it.
Physical damage. A converter that has been overheated once is brittle; a hard bump or thermal shock from driving through deep water cracks the substrate, and the rattle begins.
Lean running with high temperatures. Prolonged engine braking on a steep grade with fuel cut, as Kia found, or over-revving, as Mitsubishi found, both raise catalyst temperatures without any fault in the converter itself.
Leaded or contaminated fuel, rare now.
What does not kill a converter: normal mileage. A 150,000-mile converter on a well-running engine usually still passes. A 60,000-mile converter behind a misfiring engine does not.
What sets P0420 on a good converter
A downstream oxygen sensor that has become too responsive, or an upstream sensor that has become too slow. The monitor compares them; either change fails it. Upstream sensors slow with age and contamination, and a slow upstream sensor makes a healthy converter look lazy.
An exhaust leak before or between the sensors. Air drawn into the exhaust at a cracked manifold, a leaking flange gasket or a rusted pipe reaches the downstream sensor and makes it read lean and active. Small leaks matter; the sensors respond to fractions of a percent of oxygen.
A converter that never reaches operating temperature. Short trips in cold climates, a thermostat stuck open (P0128 in the history), or a leaking exhaust upstream that cools the gas.
A mixture fault the module has not yet flagged. Fuel trims near their limits from a small vacuum leak or a marginal sensor shift the monitor's baseline before P0171 or P0172 set.
A calibration that flags a converter still within limits, as on the Acura and Ford above.
An aftermarket converter with too little precious metal. Universal-fit converters built to a lower loading than the original often pass for a few months and then set P0420 as their small storage capacity ages. They also may be illegal to install on vehicles certified to California standards. This is the second most common way the code returns after a "fix."
A diagnostic order that ends with the converter, not starts with it
- Record everything and read the history. Whether P0420 is current, pending or history; whether misfire, lean, rich, oxygen sensor, thermostat or EVAP codes have been stored recently; and the freeze frame. A misfire or mixture code in the history changes the diagnosis before a single measurement is made.
- Check for a bulletin on the exact vehicle. On the vehicles listed above the procedure is decided.
- Look at the exhaust. Manifold cracks, flange gaskets, the pipe ahead of the downstream sensor, heat shields. Listen for a leak at cold idle with a hand near the joints. A rattle from the converter with the exhaust tapped is a broken substrate and the diagnosis is over.
- Watch both oxygen sensors live, engine warm, steady 2,000 rpm. Upstream should swing quickly between roughly 0.1 and 0.9 V (or, on a wideband sensor, a steady lambda); downstream should sit relatively flat around 0.6 to 0.7 V. Downstream swinging in step with upstream is what the code says; downstream swinging with an upstream that is slow or stuck is a sensor problem, not a converter problem. Snap the throttle and watch the downstream sensor respond and recover.
- Check fuel trims and mixture. Long-term trims at idle and cruise near zero mean the mixture is right. Trims far positive point at a leak or a weak injector; far negative point at rich. Fix the mixture before judging the converter.
- Check for misfire with the misfire counters on the scan tool, even without a misfire code. A cylinder counting a few hundred misfires per drive without setting a code is enough to harm a converter over months.
- Test the converter directly if the sensors and mixture pass: an infrared thermometer on the inlet and outlet pipes with the engine warm (the outlet should read noticeably hotter than the inlet, roughly 50 °C or more, when the converter is working), and a back-pressure test at the upstream sensor port (more than a few psi at 2,500 rpm indicates a plugged substrate).
- Replace the converter only after the cause of its death has been found and fixed, with a converter certified for the vehicle (California-certified where required), and then run the catalyst monitor with the scan tool's drive cycle so the readiness monitors complete before an inspection.
The fixes that are not fixes
Oxygen sensor spacers and "defoulers." A spacer pulls the downstream sensor out of the exhaust stream so it sees a flatter signal. It fools the monitor and does nothing for emissions, is illegal on road vehicles in the United States, and fails a visual inspection where one is done.
Clearing the code and driving to the inspection. The catalyst monitor is one of the readiness monitors an inspection requires to be complete; a cleared code with an incomplete monitor is a rejection.
Catalyst cleaner additives. They do not restore ceria that has been contaminated by oil or melted by heat. They can help marginally with light carbon on a converter that was never really failed, which is why some owners report a temporary clear.
A universal converter without the original's loading. See above; it returns.
What P0420 does not tell you
It does not tell you the converter is bad. It tells you the downstream sensor's signal looked too much like the upstream sensor's during the test. A slow upstream sensor, an exhaust leak, a mixture fault, a cold converter, a calibration set too tight and a dead converter all produce that result, and the manufacturers' bulletins show every one of them in the field. It does not tell you why a dead converter died, and a converter replaced without that answer dies again. The general principle is covered in what a trouble code does not tell you.
Frequently asked questions
Can I drive with a P0420 code?
Yes. With P0420 alone the engine runs normally; the consequences are a failed emissions inspection and a check engine light that stays on. Stop and diagnose sooner if the light flashes, the exhaust smells of rotten eggs, or the exhaust rattles, because those mean the converter is being damaged or is breaking up.
Is P0420 always the catalytic converter?
No. The code means the catalyst monitor's comparison of the two oxygen sensors failed. A slow upstream sensor, an exhaust leak ahead of the downstream sensor, a mixture fault, a converter that never warms up, or a calibration set too tight fail it too. Honda, Ford, Kia and Hyundai have each issued software for this code, and Volkswagen and Jeep pair a converter with a calibration or an engine repair rather than replacing the converter alone.
Will replacing the converter fix P0420?
Only if the converter has actually failed and the cause of its failure has been fixed. A converter killed by a misfire, a rich mixture or oil consumption dies again behind the same engine. Find the cause first, then fit a converter certified for the vehicle.
Why did P0420 come back after a new converter?
Three common reasons: the cause of the original failure (misfire, mixture, oil) was never fixed; the replacement was a universal converter with less precious metal than the original, which ages into the code within months; or the fault was never the converter, and the slow upstream sensor or exhaust leak is still there.
Do oxygen sensor spacers fix P0420?
They hide it. A spacer pulls the downstream sensor out of the gas stream so it reads flat, which fools the monitor without changing emissions. It is illegal on road vehicles in the United States and does not survive an inspection where the exhaust is looked at.
How much does it cost to fix P0420?
An exhaust leak repair or an oxygen sensor is a moderate cost. A software update is an hour at a dealer, and on the vehicles with bulletins it may be covered. A converter is the expensive outcome: an original-equipment or California-certified converter on a modern vehicle is a major part, and on many engines the converter is built into the exhaust manifold. That cost is why the diagnosis above ends with the converter rather than starting with it.
Related codes
- P0430: catalyst efficiency below threshold, bank 2. The same test on the other bank; both at once points at something shared, such as a mixture fault, oil consumption or a calibration.
- P0300 to P0308: misfire codes, the most common converter killer.
- P0171 / P0172 / P0174 / P0175: mixture codes; a rich bank overheats its converter, a lean one changes the monitor's baseline.
- P0131 to P0141: upstream and downstream oxygen sensor circuit and heater codes; the sensors the monitor depends on.
- P0128: thermostat below regulating temperature; a converter that never gets warm cannot pass the test.
- P0442 / P0455: EVAP leak codes, which on some calibrations block the catalyst monitor from running.
The code number itself tells you the family and the domain; how to read an OBD2 code number explains what each character means.