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The Complete Guide To Diagnose & Fix P0420 Yourself

Click below to watch the video and follow the steps to diagnose and fix your P0420 code. Don’t have FIXD yet? Click here to save 67% for a limited time.

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With the FIXD Sensor and app, you can view your car's live OBD2 data to pinpoint issues, see the most likely repair for P0420, get expert help from our Mechanic Hotline, and more. Click below to save 67% now. 


How To Find The Cause Of Your 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.

With a FIXD Sensor and Premium trial, you can also see the most likely repair and cost for your exact car in the FIXD app. If you want detailed engine diagnostics, cost estimates, live OBD2 data, access to our Mechanic Hotline, and more, try FIXD today for just $19.99.

Are there any other error codes?

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 like FIXD.

Unlike other cheap car scanners, FIXD isn’t a bulky handheld device that just spits out a code you have to Google for actual information you can use.

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!

Are there leaks in your exhaust system?

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!

Are your O2 sensors or catalytic converter damaged?

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 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.

How to fix an exhaust leak

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!

How to replace O2 sensors

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! 

How to replace your catalytic converter

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!

Try FIXD for just $19.99 today!

With the FIXD Sensor and app, you can see what your check engine light means, how to fix it, and see how much it should cost to fix. Click below to get FIXD, including a 14-day trial of FIXD Premium, for just $19.99 and save $1000s on repairs!

  • Read and clear check engine codes

    Forget wasting hours Googling engine fault codes. FIXD gives a detailed report of what’s wrong in simple terms, right on your smartphone, and lets you clear codes with the touch of a button.

  • View live OBD2 data

    Ready to maximize your savings with DIY repairs? Use FIXD to view live OBD2 data to test O2 sensors, pinpoint problems and fix them like a pro.​

  • Talk to a certified mechanic

    You'll also get access to our Mechanic Hotline, where an ASE-certified mechanic is available 5 days a week to offer you trustworthy, professional advice and walk you through DIY repairs.

  • Know how much repairs will cost

    With Confirmed Fix & Cost, you'll see the most likely repair for your car's check engine light, plus how much it should cost. Fix it yourself or take it to a shop knowing you can't get ripped off.

  • Predict future car problems

    Hate surprise repairs? Our Issue Forecast feature can predict which problems your vehicle is most likely to have in the future so you'll be prepared.

  • Buy your next car with confidence

    Shopping for a used car? Get the full history of any vehicle before you buy, including accident history, previous owner info, & more with our FREE Vehicle History Report.

  • Test emissions at home

    Save time and money and be confident you’ll pass emissions testing the first time with our at-home Emissions Precheck.

Real reviews from real drivers

Sarah B.
Sarah B.
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The main use is the cost estimates for everything. It’s powerful to be able to hold that up to the mechanic and be like ‘wait a second, that estimate is way over.'
Jennifer G.
Jennifer G.
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My family has saved thousands of dollars in labor alone. Thank you! Over 6 different vehicles have been fixed using FIXD Premium.
Travis W.
Travis W.
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Love FIXD Premium. Went to a mechanic and he told me I needed much more done to my Hyundai and I told him to fix only what the app said to repair. He then asked how I know, I showed him the FIXD recommended repair and cost. Never went back.
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This has been a big money saver for me. I was having car problems and I took it to the shop. $500 to fix the car. I decided to buy this before doing the repairs and glad I did. The problem was just a mass air flow sensor. Part was only $90 and I went on YouTube and fixed it in 15 minutes with a screw driver.
Noel Williams
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If you own a vehicle you absolutely have to get one, I put in the info and it told me exactly what the problem was and rated the seriousness of it, and provided locations where I could get it fixed without getting overcharged.
C. Pereira
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Easily top 3 if not top product I have ever purchased on Amazon. Easy to set up, install, and use. The premium service is excellent with a 24/7 mechanic helpline (who went above and beyond). I had a check engine light come in with a throttle control motor specifies as the issue. In total I spent $50 for the Device, $9 for a month of premium, and $20 on a relay. Problem solved (after talking it through with the call in mechanic). I spend $79 total, or could pay $120 for a diagnostic, and still not resolve my issue. 100% would purchase again and recommend.
Gretchen M.
Gretchen M.
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I called the mechanic hotline and believe you me, my money... was well spent. I spoke to John and he was a gentleman. Didn't talk down to me... answered my questions and told me to never hesitate to call for any reason about the car. Thank you so much for the ability to call a mechanic when I need help. I appreciate it and believe you me I will tell everyone what a class A company FIXD is.