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. There are many variables that could cause this code. You can take your car into a shop, or save money by learning how to diagnose and fix your problem yourself with our P0420 guide.
- One of the top 5 most common causes of a check engine light in 12 out of the 14 Honda models we have data on.
How To Find The Cause Of Your Honda P0420
The most common cause of a P0420 code is a worn catalytic converter, but this can differ depending on your specific model. These can cost over $1000 to replace, so before you replace it, make sure it really needs to be replaced. There are 4 main causes of P0420 that we need to check. Click on sections below to learn more about each possible cause.
You can use the FIXD Sensor and app to diagnose and fix P0420 yourself, see the most likely fix and cost for your Honda, access our Mechanic Hotline, and much more. Click here to try FIXD for just $19.99.
Difficulty Level: Beginner
Tools Needed: OBD2 Scanner
P0420 may not be your only code. Before you fix anything related to P0420 make sure you don’t have any other error codes because they could be the cause of your P0420. For instance, if a P0300 (cylinder misfire) or P0172 (too much fuel in air/fuel mixture) can cause a catalytic converter to fail. If you replace your catalytic converter without fixing the root cause, your new catalytic converter will fail too.
The only way to know if you have other codes is to use an OBD2 scanner. There are a few options for this:
Get a scan at AutoZone
Pros: The main benefit of this is that you get your answer for free. They will scan your car and let you know all of the codes on your car.
Cons: The downside is they won’t tell you what is causing your error codes. You’ll still have to go to a mechanic or figure that out for yourself. If you want them to clear your check engine light, they might not be willing to do that for liability reasons.
Get a scan at a Mechanic
Pros: A mechanic will be able to scan your car and will be able to figure out what is the root cause of your problems and fix it. Mechanics usually charge about $100 for a diagnosis, but they typically deduct this cost from your bill if you get them to fix the problem too.
Cons: This is probably the most expensive option. Not only will you pay for the parts, but you’ll have to pay for labor.
Scan your car with a FIXD Sensor
Pros: With FIXD you’ll be able to scan your codes yourself. You’ll also be able to read live data from your car’s sensors to diagnose your problems. If you need any help or expert advice you can call the FIXD Mechanic Hotline for free to get specific answers for your car’s problems.
Cons: FIXD isn't free, but it's only $19.99 for the Sensor. With the pocket-sized FIXD Sensor and app, you’ll be able to instantly translate your check engine light into simple terms on your phone. It tells you what it means, how serious it is, and potential fixes with diagnostic steps. You’ll also be able to read live data from your car’s sensors to diagnose your problems. With FIXD Premium, you can even call the FIXD Mechanic Hotline to get specific answers for your car’s problems. Click here to get FIXD for just $19.99 for a limited time!
Difficulty Level: Beginner
Tools Needed: Jack Stand Or Ramps
Exhaust leaks can cause P0420 codes and are relatively easy to identify. Before you start, make sure your car has been off for several hours or you can get burned by the exhaust system. You may need to raise your car on stands or ramps to access your exhaust system. Leaks can happen at the exhaust manifold flange, O2 sensor threads, pre- and post-cat exhaust clamps, and any obvious damage to the catalytic converter itself. There are a few methods you can try at home to find exhaust leaks.
Listen for exhaust leaks
Exhaust leaks have a distinctive sound. Listen for a sputtering noise shown in the video below.
Look for exhaust leaks
You can usually spot leaks as excessive black carbon buildups that are usually formed at a junction in the exhaust pipe. Make sure to check the catalytic converter and O2 sensors, including the wiring. If you can hear a rattle when knocking on the catalytic converter or if you can see obvious damage to the O2 sensor wiring, you should start by addressing these issues first. Damaged wiring might be burned, smashed, exposed, or completely severed.
Have a FIXD mechanic help you find exhaust leaks
If you’re not sure you found a leak or need advice, a FIXD Mechanic can work with you to find a leak. You can call FIXD mechanics for free with a free trial of FIXD Premium. Click here to get FIXD for just $19.99, including a free 14-day trial of FIXD Premium!
Difficulty Level: Beginner (With FIXD) – Advanced (With Multimeter)
Tools Needed: FIXD Sensor Or Multimeter/Butane Torch
Your exhaust system has two O2 sensors (oxygen sensors), one before the catalytic converter (O2 Sensor 1) and one after the catalytic converter (O2 Sensor 2). If they are working properly O2 sensor 1 should constantly fluctuate between 0.1V – 0.9V and O2 sensor 2 should be steady. Checking your O2 sensors will tell you if you need new O2 sensors or a new catalytic converter. If you can’t get a reading from your O2 sensors it means you need to replace the sensors. If the sensors are functioning and displaying unexpected readings, the catalytic converter might be worn. There are two ways to check your O2 sensors.
Testing O2 sensors with FIXD (Easy)
The easiest way to read your O2 sensors is by using FIXD. The FIXD Sensor plugs into your OBD2 port (usually under your steering wheel) and reads live data from your car’s sensors (see image below for example of O2 sensor 2 reading from FIXD app). Simply go to the “Live Data” tool in the FIXD app and pull up your O2 sensors. If you can’t get a reading from your sensors it means your sensors may be damaged. If your O2 sensor 2 reading is fluctuating like O2 sensor 1 it means your catalytic converter is bad and needs to be replaced. Click here to get FIXD for just $19.99 (normally $59).
Testing O2 sensors with multimeter (Difficult)
This is a more advanced method. You’ll first have to remove your O2 sensor from your car. Using a digital multimeter and butane torch, measure the sensor’s voltage as you heat up the element. If the voltage doesn’t get within 0.1V – 0.9V you have a bad O2 sensor and need to replace it. See the video below for guidance.
How To Fix Your Honda P0420
Before you fix P0420, make sure you correctly diagnose the cause of the code first. If you don’t, you could end up spending $1000s buying a new catalytic converter when that wasn’t the cause of the code. Below are some of the main causes of a P0420 code. Click on sections below to learn more about each repair.
Difficulty Level: Intermediate
Tools Needed: Tail Pipe Expander, Exhaust Couplers, Exhaust Clamp, Exhaust RTV
If you’ve found the leaks in your exhaust the next step is sealing them. You can do this with a welder, but if you don’t have one, there are other ways. The easiest alternative is to simply remove the damaged length of the exhaust and replace that length with an exhaust coupler. Watch the video below to see how it’s done.
If you need advice, a FIXD Mechanic can work with you to fix the leak. You can call FIXD mechanics for free with FIXD Premium. Click here to get FIXD for just $19.99, including a free 14-day trial of FIXD Premium!
Difficulty Level: Intermediate
Tools Needed: O2 Sensor Socket, New O2 Sensor
If you’ve used the FIXD Sensor to test your O2 sensors and discovered they are faulty and need to be replaced, you can do this yourself. You will need an O2 sensor socket to remove the old sensor and install the new one. An O2 sensor socket is a 22mm socket that is designed to not get in the way of the wires coming out of the O2 sensor so you don’t damage the new O2 sensor. Watch the video below to see how it’s done.
If you need advice, a FIXD Mechanic can work with you to replace the sensor. You can call FIXD mechanics for free when you’re a member of FIXD Premium. Click here to get FIXD for $19.99, including a free 14-day trial of FIXD Premium!
Difficulty Level: Intermediate (with direct fit catalytic converter) – Advanced (with universal fit catalytic converter
Tools Needed: New Catalytic Converter, Socket Wrench, Penetrating Oil
Catalytic converters can be relatively easy to replace depending on what type they are. Direct fit catalytic converters are designed specifically for your car and will bolt right into your exhaust system. Direct fit catalytic converters can be expensive because they are made to fit one model. Universal fit catalytic converters are cheaper and are designed to fit a wide range of cars. You will have to custom fit a universal catalytic converter into your car. If your goal is to save money and you aren’t afraid to work on your car, you should consider a universal fit catalytic converter. Watch the video below to learn how to replace your catalytic converter.
If you need advice or help, a FIXD Mechanic can work with you to place a new catalytic converter. You can call FIXD mechanics for free when you’re a member of FIXD Premium. Your FIXD Sensor comes with a free 14-day trial of Premium. Click here to get your FIXD Sensor today for just $19.99!
Honda P0420 Symptoms
- Check Engine Light is on
- Lack of power from the engine
- Decreased fuel economy
- Rotten egg or a sulfur smell
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.
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.
- Air fuel sensor $200-$300
- Oxygen sensor replacement $275-$500
- Catalytic converter $400-$2400
- A leak in exhaust $100-$200 (if welded to repair)
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. If you want a step by step guide on how to diagnose and fix this problem, check out our P0420 guide.
DIY difficulty level: Advanced
This repair requires mechanical knowledge and is not recommended for beginners.
Tools/parts needed (our top picks from Amazon):
STEP 1: USE FIXD TO ENSURE NO OTHER ENGINE CODES ARE PRESENT.
Use FIXD to scan your Honda to verify P0420 is the only code present. If other codes are present, they must be addressed first.
STEP 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.
STEP 3: CHECK THE DOWNSTREAM O2 SENSOR.
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. Don’t have a FIXD Sensor yet? Click here to get yours for just $19.99, including a free 14-day trial of FIXD Premium!
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Fixed It But The Check Engine Light Is Still On?
Check engine light sometimes need to be reset manually, check out our article:
How to Reset Your Check Engine Light | 4 Ways To Clear It (With or Without a Scanner)
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.
