P0420: Catalyst System Efficiency Below Threshold Bank 1
A generic powertrain (engine and transmission) trouble code, same meaning on every brand.
Yes - the car remains driveable. Fix it before emissions testing: this code is an automatic inspection failure in most states.
Symptoms
- Check engine light with no drivability change (most cases)
- Slight loss of power in severe cases
- Rotten-egg smell from exhaust
- Failed emissions inspection
Likely causes, most common first
- Catalytic converter worn below efficiency threshold (most common on 120k+ miles)
- Lazy or contaminated downstream oxygen sensor
- Exhaust leak upstream of the rear sensor skewing readings
- Long-term misfires or oil burning that poisoned the converter
Read P0420 yourself before paying for diagnosis
A basic Bluetooth OBD-II scanner ($20-40) plugs into the port under the dashboard and shows this code with live sensor data on your phone - useful both for confirming the fix and for talking to a shop from an informed position.
What is going on and how it gets diagnosed
P0420 says the catalytic converter on bank 1 is not cleaning exhaust as efficiently as it should. The computer compares oxygen sensor signals before and after the converter; when the rear signal starts mirroring the front one, the converter is no longer storing oxygen - the chemistry inside is exhausted.
Before paying for a converter, rule out the cheap causes: a tired rear O2 sensor or a small exhaust leak can fake this code. If the converter does need replacement, ask what killed it - converters rarely die on their own before 150k miles. Unfixed misfires or oil consumption will poison a new one just as fast.
Repair options and cost
Typical US independent-shop ranges, parts and labor. Dealers run higher; regional labor rates vary.
Owner reports mentioning P0420
“I had my purge valve replaced under 24N07 ford costumer satisfactory program and the faulty purge valve damaged my catalytic converter and caused my engine light to illuminate with P0420.”2013 Ford Focus · May 2026
“2019 Honda Odyssey purchased new and one owner. Vehicle's VIN didn't fall under Honda TSB # 21-010 despite vehicle suffered a known defect pattern so Honda denied assistance soley due to VIN exclusion. Sequence: emissions system warning, P0420, injector failure diagnosis, consequential catalyst failure after injector malfunction.”2019 Honda Odyssey · May 2026
“Multiple times with 6 months my check engine light comes on but no code is detected. However, when tested it tells me to look out for code P0420. The issues I have encountered: - High RPM of 2,000 when vehicle is started - Check engine light with no code. Light comes and goes on it own - Car jolts and vibrates high while idling - Car eats oil like no tomorrow. Having to do an oil change every 3,000 miles”2014 Kia Soul · May 2026
Show 7 more owner reports
“My car burns oil very fast. I am putting in more than 5 quarts of oil in between oil changes. I have replaced the flex pipe, the catalytic converter two times, and both the upstream and downstream oxygen sensors. The catalytic converter now needs to be replaced again after only 7 months after being installed. The car keeps getting P0420 codes. I found out this last converter was bad while on the highway going 70 mph up a hill. The speed suddenly decreased all the way down to 50 mph which put me in great danger of getting hit from behind at high speeds. It would not go any faster..”2014 Kia Soul · Apr 2026
“Car uses excessive amount of oil! We have had the timing replaced and now it's stating the P0420 catalyst system efficiency below threshold bank 1.”2015 Chevrolet Equinox · Mar 2026
“I have a code of P0420 show up on a vehicle diagnostic test. This is directly related to recall SA363 ECU LOGIC . The engine light kept coming on. I was driving on the highway and the car stalled and then stopped as I was coming off the exit.”2019 Kia Soul · Mar 2026
“I see there was a P0420 error code related to catalytic converter oil consumption recall, but when I put in my VIN it said no recalls. This is for a 2020 Hyundai Elantra 2020. The problem comes and goes and was able to be reproduced or confirmed by a dealer Hyundai. The warning lamps, messages comes on frequently and was reportedly fixed charges $2000 for a sensor. I the code and it is all over the internet that it is a recall.but does not show up with VIN.i need assistance with getting this fixed at the dealer”2020 Hyundai Elantra · Mar 2026
“My car has 2 codes, P285 and P0420. When i drive the car and accelerate the car starts to skip. At 1 point in time i was at a stop with my feet on the breaks and i tried to go once the light was green the car was not accelerating.!”2014 Nissan Versa Note · Feb 2026
“Dear Sir, On April 2026, I need to do the semiannual smog check. Right now, I face DTC P0420 error code and I can't pass the smog check inspection. Note that I don't have the engine light on indicating the a/m error code. Only my OBD2 shows the error DTC Code P0420. I did some searhing and noticed that the ECM for Sienna should be updated to erase the a/m DTC code. I would appreciated to help me how the resolve this issue ASAP. It will enable me to renew my registration.You may contact on my cell phone 747 217 7254 or my email at massey.miri@gmail.com. Regards, Massey”2002 Toyota Sienna · Feb 2026
“My vehicle has P0420 and P0430 code errors, indicating a problem with the catalytic converters/emission system. There is a current recall on 2018 VW Atlas vehicles for this issue, but it does not include my vehicle. It appears this recall needs to be extended as the same issue is occurring on other 2018 VW Atlas vehicles.”2019 Volkswagen Atlas · Nov 2025
Cars that report P0420 most often
Based on mentions of this code in NHTSA owner complaints.
Frequently asked questions
Can I drive with P0420?
Yes - the car remains driveable. Fix it before emissions testing: this code is an automatic inspection failure in most states.
How much does it cost to fix P0420?
Typical US repair costs range from $100 for the simplest fix (downstream o2 sensor) to $2,800 in the worst case. The right number depends on which cause your car has - see the causes list.
Definition per SAE J2012 standard nomenclature. Symptoms, causes and costs are editorial guidance - always confirm with a diagnosis of your specific vehicle.