P0420 Catalytic Converter Efficiency Low (Bank 1)
Drive normally - check soon
Your engine computer says the catalytic converter on bank 1 no longer cleans the exhaust well enough. It is often a worn converter, but check the cheap causes first: an exhaust leak, a lazy rear oxygen sensor, or an engine fault that is hurting the converter. Safe to drive for now. Fix it before your emissions test.
- Safe to drive?
- Yes, for now
- Typical cost at a shop
- $125-$2,480 RepairPal national averages, parts and labor
- Emissions test
- Yes, while the check engine light is on.
- Carmaker bulletins
- 6 notes with links
What it means
Your car has two oxygen sensors on bank 1: one before the catalytic converter and one after it. A good converter stores oxygen and evens out the exhaust, so the rear sensor stays fairly calm. When the rear sensor starts to follow the front one, the converter is not doing its job, and the computer sets P0420. Bank 1 is the side of the engine with cylinder 1. A four-cylinder engine has only bank 1.
The technical version
The catalyst monitor runs when the engine is warm, in closed loop and at a steady speed. It compares the rear (downstream) sensor with the front (upstream) sensor, or measures how much oxygen the converter can store. If the result is past the limit the carmaker set, the test fails. On many cars the first failure stores a pending code, and the light comes on only if the test fails again on a later drive. On many newer cars the front sensor is a wide-band air-fuel sensor; our own 2010 Accord has one. It reports lambda or a current, not a 0.1 to 0.9 V swing, so the 'rear copies front' picture looks different on those cars (see the chart from our own Accord below).
Official name: Catalyst System Efficiency Below Threshold (Bank 1).
Symptoms
- Often none. The car drives the same and the only sign is the check engine light.
- A failed emissions or smog test.
- A rotten-egg (sulfur) smell from the exhaust on some cars.
- A rattle under the car if the inside of the converter has broken up.
- Weak power or poor fuel economy, but only if the converter is also clogged. That is less common and usually comes with other codes.
Causes, most likely first
| Cause | How likely | How to tell |
|---|---|---|
| A worn catalytic converter | most common | No other codes, fuel trims near normal, and the rear sensor moves almost as much as the front one at a steady cruise. |
| An engine problem that is hurting the converter: misfires, a rich mixture, or burning oil or coolant | common | Misfire codes or misfire counts, long-term fuel trim far from 0%, blue smoke, or oil or coolant that goes down between changes. Fix this first or a new converter will fail too. |
| An exhaust leak before or near the rear sensor | common | A ticking or puffing sound on a cold start, soot at a joint or the flex pipe. Outside air makes the rear sensor read lean. |
| A slow or failed rear oxygen sensor | less common | The rear sensor never goes high when you accelerate hard or never goes low when you coast. Sensor codes such as P0136 to P0141 often come with it. |
| A low-grade replacement converter | less common | The converter was replaced before, and P0420 came back within months. |
How likely is our ranking from repair data and common practice, not a published rate.
What the live data shows
Two readings tell most of the story: the front and rear oxygen sensors on bank 1, read while the engine is warm and in closed loop. The rear sensor should change slowly. When it drops to the low end and back again and again, the converter is not storing oxygen. Long-term fuel trim, misfire counts and the freeze frame then tell you whether the engine is the reason.
On our own 2010 Accord with P0420 stored, the rear sensor crossed 0.45 V 18 times in 163 warm seconds, with no fuel cut.
- This is our own 2010 Honda Accord 2.4. P0420 was stored in the engine computer when we plugged in.
- Grey line: the front sensor. On this engine it is a wide-band air-fuel sensor, so it reads in lambda, not volts. 1.00 is the ideal mixture.
- Amber line: the rear sensor, after the catalytic converter. It crossed 0.45 V 18 times in 163 seconds once the engine was warm.
- The engine stayed in closed loop the whole time. There was no fuel cut, so the drops are not the injectors switching off.
- A healthy converter stores oxygen, so its rear sensor usually changes slowly. Here it keeps dropping to the low end and back.
How we counted
From the first second coolant read 75 C (167 F) to the first decel fuel cut. That is 163 seconds. About 1 reading per second per sensor. Timestamps are whole seconds. Exact duplicate rows dropped; one reading per sensor per second kept (the last). A crossing is the rear voltage moving from one side of 0.45 V to the other between two kept readings. Counted another way, the same drive gives more: 24 crossings when every raw reading is kept, 22 when the first reading in each second is kept. We show the lowest count.
What this chart does not show
- The app read each sensor about once per second. A sensor can switch faster than that, so the real number of crossings can be higher. Treat the count as a floor.
- Some of the drops came while coasting with the throttle closed. A lean exhaust while coasting is normal, and a long coast can pull even a good converter's rear sensor low.
- One short drive (about 1.4 km) is not a diagnosis. The engine computer's own catalyst test is what stored P0420; this chart shows what that looks like from the outside.
- Long-term fuel trim held between -18% and -16% for the whole window: the computer was taking fuel away. We have not found the cause yet. Rich running is one known way converters get damaged, so it is worth checking before a new converter goes in.
- The check engine light was off during this drive, with P0420 still stored. Stored codes can stay in memory after the light goes out.
On every scan from September 21 to 24, 2026, the stored result of the car's own catalyst test was 2.785 against a limit of 3.0: a pass, but close to the limit. A converter near its limit can pass on some drives and fail on others, which is one reason P0420 comes and goes.
What to check first, in order
-
Read every code, pending codes and the freeze frame.
Misfire codes (P0300 to P0308) or lean and rich codes (P0171, P0172) are often the reason the converter failed. Fix those first.
What the data shows: The freeze frame shows the speed, load and coolant temperature when P0420 set. Misfire counts, where the car reports them in Mode 06, show whether any cylinder misses.
Cost: no cost if you have a scanner
-
Check your car for a bulletin or a warranty before you buy parts.
Several carmakers fix P0420 with a software update or cover the converter for longer (see make notes). The federal emissions warranty also covers converters for 8 years or 80,000 miles.
What the data shows: Not a scanner step. A dealer can check your VIN.
Cost: free
-
Check long-term fuel trim with the engine warm.
A mixture problem can damage the converter and can also confuse the test.
What the data shows: Near 0% is normal. A large, steady number means the computer is correcting a real mixture problem. Our own Accord held about -17%, so we are checking that before we price a converter.
Cost: no cost if you have a scanner
-
Look and listen for exhaust leaks from the engine back to the rear sensor.
A leak lets outside air reach the rear sensor and can fake a bad result.
What the data shows: The rear sensor reads low (lean) for long periods while the front sensor sits near the ideal mixture.
Cost: free to look; a weld or gasket if you find one
-
Watch the front and rear sensors at a steady 30 to 60 mph, engine fully warm.
This is the same picture the car's own test uses.
What the data shows: With a good converter the rear sensor changes slowly. With a worn one it keeps swinging, often across 0.45 V. On our Accord it crossed 0.45 V 18 times in 163 warm seconds.
Cost: no cost if you have a scanner
-
Test the rear sensor itself.
A lazy sensor can set P0420 on a good converter.
What the data shows: Accelerate hard for a moment: the rear sensor should rise to about 0.8 V or more. Coast with your foot off: it should fall near 0.1 V. A sensor that cannot reach both ends is suspect.
Cost: $440-$523 at a shop (Oxygen sensor replacement, RepairPal)
-
Replace the converter, only after the steps above.
It is the expensive part, and a new one fails again if the engine cause is still there.
What the data shows: After the repair, the catalyst monitor shows 'complete' and P0420 does not come back as a pending code.
Cost: $2,280-$2,480 at a shop (Catalytic converter replacement, RepairPal)
Is it safe to drive?
Drive normally - check soon
Yes, for now. P0420 is an emissions fault. It does not stop the engine or hurt it by itself.
The car drives normally and you are not doing sudden damage by using it. It will fail an emissions test until it is fixed. If the converter was damaged by misfires or a rich mixture, fix that cause first. If P0420 comes with misfire codes or a flashing check engine light, treat the misfire as urgent.
Stop driving and get help if
- The check engine light flashes: that is an active misfire that can overheat the converter.
- The engine shakes, stumbles or loses power.
- You smell strong sulfur or burning, or hear a loud rattle under the car.
What it costs
| Repair | Typical cost |
|---|---|
| Check engine light diagnosis (shop)RepairPal, updated September 3, 2026 | $125-$183 |
| Exhaust manifold gasket replacementRepairPal, updated September 3, 2026 | $417-$596 |
| Oxygen sensor replacementRepairPal, updated September 3, 2026 | $440-$523 |
| Catalytic converter replacementRepairPal, updated September 3, 2026 | $2,280-$2,480 |
National averages for the full job at a shop, parts and labor, from RepairPal (updated September 3, 2026). Your price depends on the car and the area. Many of the first checks on this page cost nothing with a scanner. CarMD found the average check engine repair in 2025 was $554, from more than 39 million US vehicles. Catalytic converter replacement was CarMD's number 1 check engine repair in 2025, at an average of $1,511. Oxygen sensor replacement was number 2, at an average of $287. CarMD averages come from a different method (repairs reported by its users), so they can sit below RepairPal's shop estimates. RepairPal notes that catalytic converters are covered by a federally required 8-year or 80,000-mile warranty from the carmaker. On a newer car, ask the dealer before you pay.
Sources for the notes: CarMD 2026 Vehicle Health Index (press release, April 22, 2026); RepairPal: P0420 code page.
Can I clear it? Will it come back?
You can clear P0420 with a scanner, but clearing also resets the readiness monitors, and an emissions test needs them to be ready. The catalyst monitor runs only under certain conditions, so it takes time: Ohio EPA says to drive at least two or three days with highway and city miles, and Georgia's program says one to two weeks including some highway driving. If the converter is still weak, P0420 comes back, first as a pending code. Clearing it the day before a test does not work.
Will it fail an emissions test?
Yes, while the check engine light is on. Texas, Arizona and Georgia all say a car with the light on fails their OBD test. A car can also be turned away if too many readiness monitors are not ready, for example right after the code was cleared: New York allows only one not-ready monitor on 2001 and newer cars.
Carmaker notes
Service bulletins and warranty extensions that name this code. Ask a dealer to check your VIN.
-
Honda some 2010 Accord coupes and sedans, 2010 Crosstour, 2010 Odyssey, 2011 Pilot, 2010 Ridgeline (by VIN range)
Honda bulletin 14-002 (February 2014): the cap of a front air-fuel sensor may fall off and bore a hole through the warm-up catalytic converter. It is listed by VIN range, so ask a Honda dealer to check yours. Our own 2010 Accord 2.4 has P0420 stored; we have not yet had it checked against this bulletin.
Honda 14-002 via NHTSA static.nhtsa.gov
-
Honda 2016-2017 Pilot, 2017 Ridgeline, 2018 Odyssey
Honda bulletin 20-100 (February 2021): engine software may read a good converter as worn, and injector wear can cause misfires. It covers P0420 and P0430 among other codes, for 10 years or 150,000 miles.
Honda 20-100 via NHTSA static.nhtsa.gov
-
Acura 2015-2017 TLX V6
Acura bulletin 19-049 (August 2019): the engine software may misread sensor inputs as a worn catalytic converter. The fix is a software update.
Acura 19-049 via NHTSA static.nhtsa.gov
-
Honda 2014-2016 CR-Z
Honda bulletin 22-040 (September 2022): sensors may read the catalyst as worn. The fix is a software update.
Honda 22-040 via NHTSA static.nhtsa.gov
-
Ford 2018-2020 F-150 2.7L EcoBoost, 2020-2021 Expedition and Navigator
Ford bulletin 22-2260 (January 2023): P0420 or P0430 may come from high sulfur in the fuel. The fix is to de-sulfate the converters, not replace them.
Ford TSB 22-2260 via NHTSA static.nhtsa.gov
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Subaru 2017-2023 models
Subaru's P0420 procedure (SIB 09-110-23R, October 2023) checks first for misfire codes, P0171 or P0172 and oxygen sensor codes, the same order as this page.
Subaru SIB 09-110-23R via NHTSA static.nhtsa.gov
Related codes
Also related, no guide yet: P0136, P0137, P0138, P0141, P0172.
How Obie handles P0420
Obie reads the stored, pending and permanent codes, the freeze frame, both oxygen sensors, fuel trims and the Mode 06 test results where the car reports them. It then tells you which of the cheap causes to rule out before you pay for a converter. Obie is part of ScanMyCar Pro. Codes and live sensors are free.
Questions people ask
How do you fix code P0420?
Find out why the converter is failing before you replace it. Fix misfires, fuel-trim problems and exhaust leaks, and test the rear oxygen sensor. If all of that checks out, replace the catalytic converter with a quality unit.
Can a bad O2 sensor cause a P0420 code?
Yes, but less often than people hope. A slow rear oxygen sensor can make a good converter look bad. Test it before you buy a converter: it should reach about 0.8 V under hard acceleration and near 0.1 V when you coast.
Is P0420 bank 1 or bank 2?
Bank 1. Bank 1 is the side of the engine with cylinder 1. The bank 2 version is P0430. A four-cylinder engine has only bank 1.
How much does it cost to fix P0420?
It depends on the cause. A scan and a gasket or sensor costs far less than a converter. See the cost table on this page for sourced national averages.
Will a catalytic converter cleaner fix P0420?
We have not tested one. A cleaner cannot restore a converter whose coating is worn out or melted, and it does not fix the engine problem that often causes P0420.
Can P0420 go away on its own?
The light can go out if the converter passes its test on later drives, and the code can stay stored for a while. That usually means the converter is near its limit, not fixed. Our own Accord shows this: P0420 stored, light off, and a catalyst test result close to its limit. Texas fails a car when the light is commanded on with a code set, so a stored code with the light off is a different case, but the next failed test can turn the light back on.
Sources
- RepairPal: P0420 code page repairpal.com
- RepairPal: Check engine light diagnosis (shop) (updated September 3, 2026) repairpal.com
- RepairPal: Exhaust manifold gasket replacement (updated September 3, 2026) repairpal.com
- RepairPal: Oxygen sensor replacement (updated September 3, 2026) repairpal.com
- RepairPal: Catalytic converter replacement (updated September 3, 2026) repairpal.com
- CarMD 2026 Vehicle Health Index (press release, April 22, 2026) prnewswire.com
- Honda 14-002 via NHTSA static.nhtsa.gov
- Honda 20-100 via NHTSA static.nhtsa.gov
- Acura 19-049 via NHTSA static.nhtsa.gov
- Honda 22-040 via NHTSA static.nhtsa.gov
- Ford TSB 22-2260 via NHTSA static.nhtsa.gov
- Subaru SIB 09-110-23R via NHTSA static.nhtsa.gov
- Texas DPS emissions testing FAQ dps.texas.gov
- Arizona myazcar.com: OBD and the check engine light myazcar.com
- New York DMV: what do you mean my car's not ready dmv.ny.gov
- Ohio EPA readiness fact sheet dam.assets.ohio.gov
- Georgia Clean Air Force: learn about OBD testing cleanairforce.com
A guide, not a diagnosis. This page describes what P0420 generally means across OBD2 cars. What is actually wrong with your car depends on its live data and its history, and any repair decision deserves a proper diagnosis, whether that is you with a scanner or a professional with a lift.
