OBD2 Codes

Honda P0420 – Meaning, Causes, Symptoms, & Fixes

P0420 catalyst system efficiency below threshold bank 1

What Does Honda Code P0420 Mean?

  • Honda P0420 definition: Catalyst system efficiency below threshold (Bank 1)
  • Issue Severity: MODERATE – Extended driving with this code can cause internal engine damage.
  • Repair Urgency: Get this code fixed as soon as possible to avoid further damage to your emission system and possible engine damage.
  • Diagnosis: The most common cause of Honda P0420 is a bad catalytic converter, but it can be caused by anything from a faulty oxygen sensor to a rich or lean running condition, or misfires. Because there are so many variables that could cause this code, the best cost savings is to take your car into a shop to have it diagnosed as quickly and accurately as possible.

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The function of the catalytic converter is to break down harmful pollutants that are created by your Honda during the combustion cycle and convert them into less harmful gases. Code P0420 indicates that the catalytic converter is not functioning efficiently, therefore increasing the output of harmful pollutants by the vehicle.

Honda P0420 Causes

The most common reason for check engine code P0420 is catalytic converter failure, but there are other potential causes for this code. It’s important to look for other codes that could indicate the root cause of converter failure, such as misfires or an incorrect air-fuel ratio.

  • Worn or internally failing catalytic converter (Most Common)
  • Faulty oxygen sensor
  • Faulty air-fuel sensor
  • A leak in the exhaust system
  • Misfire (the root cause of converter failure)
  • Rich or Lean air-fuel ratio (the root cause of converter failure)
  • Leaded Gasoline (the root cause of converter failure)

Honda P0420 Symptoms

  • Check Engine Light is on
  • Lack of power from the engine
  • Decreased fuel economy
  • Rotten egg or a sulfur smell

How Do I Fix Honda Code P0420?

The first step is to get it diagnosed to figure out what is causing the catalytic converter fault. 

If you’re having catalytic converter issues and you’re not comfortable diagnosing this issue at home, we recommend finding a RepairPal certified shop nearby to pinpoint the problem and give an accurate estimate for repairs. 

These shops can not only help you figure out what’s going wrong before you waste time and money on the wrong parts, but they also offer a minimum 12-month, 12,000-mile warranty and stand behind all their estimates with guaranteed fair pricing.

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How Much Does It Cost to Fix Honda Code P0420?

P0420 can be caused by anything from a bad sensor to a failed catalytic converter. It’s impossible to give an accurate estimate without properly diagnosing the issue first. 

If you take your car to a shop for diagnosis, most shops will start with an hour of “diag time” (the time spent in labor diagnosing your specific issue). Depending on the shop’s labor rate, this typically costs somewhere between $75-$150. Many, if not most, shops will apply this diagnosis fee to any required repairs if you have them perform the repairs for you. From there, a shop will be able to give you an accurate estimate for repairs to fix your P0420 code.

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Possible Repair Costs for Honda P0420

Once properly diagnosed, P0420 may require one or more of the following repairs to resolve the underlying issue. These prices are based on national averages and include parts and labor. Your cost may differ depending on your location and type of vehicle.

DIY Steps to Diagnose Honda Code P0420

If you’d like to try to fix code P0420 at home without throwing money at parts, you’ll want to follow the steps below for proper diagnosis. Keep in mind this is an advanced-level diagnosis and repair and not recommended for beginners. Diagnosis can be a time and labor-intensive process for inexperienced DIYers.

DIY difficulty level: Advanced

This repair requires mechanical knowledge and is not recommended for beginners.

Tools/parts needed (our top picks from Amazon):


Use FIXD to scan your Honda to verify P0420 is the only code present. If other codes are present, they must be addressed first.


Pay close attention to the exhaust manifold, gaskets, and exhaust pipes pre-catalytic converter. If any leaks are found, repair the leak, clear the code, and complete several drive cycles to verify that was the fix.


With the vehicle running and at normal operating temperature, use the FIXD live data feature or a digital multimeter to check the voltage reading of the downstream O2 sensor. The downstream O2 sensor produces a relatively steady voltage reading of approximately 0.45V if the catalytic converter is functioning properly. If the voltage of the downstream O2 sensor is constantly jumping between 0.1V and 0.9V, the catalyst is worn and the catalytic converter needs to be replaced.

Common P0420 diagnosis mistakes

Oftentimes, this code is thought to be a faulty O2 sensor or A/F Sensor. While this is a possibility, the most common problem is a faulty catalytic converter. Do not overlook other codes that are paired along with P0420. Codes like P0300, P0301, P0302, P0303, P0304, P0305, P0306, P0307, and P0308 are misfire codes, and these codes require misfire diagnosis. If the catalytic converter is replaced without repairing the misfire, the new catalytic converter will go bad again. Also, make sure you don’t have codes P0174, P0171, P0172, or P0175 which can mean the engine is running rich or lean, which can burn out your catalytic converter. If these codes or any other codes are present, they should be addressed first.

Still Need Help Fixing Honda Code P0420?

If you’ve followed the steps above and are still experiencing Honda check engine code P0420, please contact the FIXD Mechanic Hotline if you’re a FIXD Premium subscriber or find a RepairPal certified shop near you for proper diagnosis and fair pricing.


Recovering autocross and track day enthusiast. Once turned a VW Jetta into a pickup truck. Lives in a van down by the river. Dream car: 2001 Subaru WRC rally car.

Justin Hughes
Recovering autocross and track day enthusiast. Once turned a VW Jetta into a pickup truck. Lives in a van down by the river. Dream car: 2001 Subaru WRC rally car.

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    1. The only code present is the P0420 with a bank 1, meaning its an O2 sensor.

      1. If the only code is PO420 with bank 1 is it O2 sensor or is it a flat in one of the injectors how do I tell if it is up stream or down stream and what does the bank 1 mean .

        1. I check all these things and replace catalytic but problem still.some body can reply properly

        2. Bank 1 is the exhaust pipe taking exhaust from the number 1 cylinder. Upstream/downstream is a reference to the exhaust gas flow in relation to the catalytic converter. Basically, upstream is the one between the engine and the catalytic converter. Downstream is the one between the catalytic converter and the muffler.

      2. My is only the code P0420 nothing else wat does this mean?

    2. The engine light is off. Could the P0420 code have been associated with the gas cap not being fully tightened?

    3. My 2008 Accord (3.5L V6) has P0171, P0174, P0157, P0420 (current), P0420 (temporary), P0430 (current) and P0430 (temporary) codes. All fingers point to catalyst + sensor but there is no decrease in power, engine sound is normal as it has been and fuel economy hasn’t noticeably worsened. Just the check engine light which is permanently on.

    4. I have a 2015 Honda Odyssey with about 115,000 miles. I was just told I have codes PO420, PO430, and PO 301.
      The engine light has been coming on and going off for the last 6 months. Finally it was blinking and I took it to mechanic.
      They say I need the front and rear catalytic converter replaced plus the first coil. $2500.
      Does this seem right? Thank you

    5. I have a 2005 Honda Pilot and my check engine light is on. I had it diagnosed and the code was P0420 Bank 1 and 2 threshold. Where do I start?

      1. I use 2011 Honda Accord, the rate at which my car consume fuel is scaring. It was scanned and diagnosed P0420 code, I have changed the catalyst and Ox sensor, the consumption still has not improved. I don’t know what else to do. Can someone advise pls, this is eating me up. I need help plsssssss

    6. I have an Element 2011 my catalytic converter was stolen and a makeshift one replaced it due to the Honda one was on back order and I was out of town. My check engine light came on a few weeks after the repair, so my mechanic systematically replaced all the sensors. Still had the check engine light on…. So a Honda catalytic converter was ordered and installed. Finally got my car back today and within an hour of running errands my check engine light is back on. Help! where do I go from here? Everything has been restored to original why would it be happening?

    7. I Have 2016 Honda Accord and it was diagnosed with P0420 code only I took it too a mechanic he said he can fix it cheap and I will pass my inspections. 3 Months later my check engine light came back on with P0420 code only I took it back to the same machine he said he’ll fix it. Im just wondering if I need replace the catalytic converter or just some cleaning is required?

    8. 011921-1 Code P0420 2007 Honda Ridgeline 3.5L
      I got the same code P0420 01-19-2020 at or around 199,650. ¾ tank left. Will get MPG at next refill.
      This is a neat experiment. 2007 ridgeline 3.5 L. with 192,958 mil. I am the only driver. Driving habit constant, no hauling or towing trailer.
      First 3 tanks of my ownership 42.31 Gallons MPG 14.10. Cause Low tier pressure and dirty air cleaner.
      Last 20 tanks average after Repair tier and replace air cleaner 17.12 MPG Best tank 19.12 MPG worst 15.35 MPG.
      Note last 3 tanks have been E-15 88, average 16.56 MPG over 58.256 Gallon. Maybe this Honda cannot run 3 tanks in a row of this stuff. I will run this tank down and refill with regular 87.

    9. I am doing and experiment to help with your low gas mileage. Have you had a repair done to your emission system such as replace O2 senser or replace catalytic converter? As mentioned in this post. If so, did your light go out and milage come back up?
      When new window sticker, if equipped with 3.5 L. should be around 23 MPG 18 city and 28 highway.
      Have you kept an accurate record of your average Mile Per Gallon prior to this issue?
      Gas mileage decrease can be caused by symptoms in this article and may be a sign of inconstant driving habits or hard driving (if you are sharing your vehicle) Low tier pressure, dirty air cleaner, idle time (remote start)

    10. I have had 2 ridgelines running first one to 360,000 miles average mpg 18-19, second one currently have at 246,000 average has been 19 mpg, 90% highway driving now dropped down to 17-18, though have front end damage and need a third one! Engine and tranny are great , don’t buy any other brand but Toyota

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