OBD2 Codes

Honda P0420 – Meaning, Causes, Symptoms, & Fixes

P0420 catalyst system efficiency below threshold bank 1
Reading Time: 2 minutes


Honda Code P0420 Definition

Catalyst system efficiency below threshold (Bank 1).

What Does P0420 Mean For Your Honda?

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 harmless gases. Code P0420 indicates that the catalytic converter is not functioning efficiently, therefore increasing the output of harmful pollutants by your Honda.

Honda P0420 Symptoms

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

Honda P0420 Causes

  • Faulty oxygen sensor
  • Worn and internally failing catalytic converter
  • Leak in exhaust system

Honda Code P0420 Severity – Moderate

While P0420 does not pose a danger to the driver and may not produce any symptoms with issues related to drivability, it can cause severe damage to other components of your Honda if not addressed in a timely manner. It is recommended that you address P0420 as soon as possible to keep the cost of the repair to a minimum.

Honda Code P0420 Common Diagnosis Mistakes

Do not overlook other codes that are paired along with P0420. Honda codes P0300, P0301, P0302, P0303, P0304, P0305, P0306, P0307, P0308, P0174, and P0171 can give false data readings that create conditions which trigger code P0420. If any other codes are present, they should be addressed first.

Honda P0420 Diagnosis

Tools Needed to Diagnose:
  1.  Scan your Honda to verify P0420 is the only code present. If other codes are present, they must be addressed first.
  2. Examine the exhaust system for damage and leaks. 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.
  3. With the Honda running and at normal operating temperature, use the 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.


You may also like


  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. 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

Leave a reply

Your email address will not be published. Required fields are marked *

More in OBD2 Codes