OBD2 Codes

Ford P0420 – Meaning, Causes, Symptoms, & Fixes

P0420 catalyst system efficiency below threshold bank 1

What Does Ford Code P0420 Mean?

  • Ford 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 Ford 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 Ford 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.

Ford 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)

Ford 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 Ford 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 Ford 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 Ford 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 Ford 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 Ford 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 Ford Code P0420?

If you’ve followed the steps above and are still experiencing Ford 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. My ‘check engine soon’ error message on my 2012 Ford Focus was resolved very easily and it did not cost me anything. My fuel tank was half full. I simply filled my gas tank to full and the ‘ check engine soon’ error message stopped and did not reappear.
      A partially full gas tank could create symptoms that there is a pressure(vacuum) leak in the fuel system and falsely trigger the ‘check engine soon’ error message. The only cost to fix the error is the price of fuel to fill the gas tank.

      1. This means that the purge valve of your Evap system is bad. It causes pressure in the tank which happens when there is less than 50% empty space in the tank. It has happened to me twice. I changed valve brands on the second replacement.

        1. Hi Ken.

          I am having the exact same issue with my 2012 Ford Focus. My BF is a mechanic and would like to know the brand you used for the 2nd replacement??

    2. For this particular make and model to pull this code, it is more than likely the recall associated with it. If your vehicle feels like a clutch is slipping, or a transmission issue, do a bit more research. I had my clutch replaced once under warranty, and convince myself that that was the issue again even though the shutter felt different. It is very confusing, and you could end up spending a lot of money that will eventually be reimbursed back to you, but it’s such a headache. The dealership did my recall, and I was still having issues. I went and got my codes pulled today, and I pulled this code associate with the recall. My CPV was replaced and my TCM was reprogrammed, but car had same issues and got worse. Broke down with dash displaying 49 miles left to empty. I was frustrated and confused until about 20 minutes ago. My car will be fixed, and I knew something wasn’t right when I was asked to pay a diagnostic fee (not that there is anything wrong or uncommon about that)

      1. was it resolved, looking at similar issues.

    3. 2011 explorer just performed an oil change, tune up and flushed my antifreeze, new flex pipe and dealer recall repair for exhaust. Had p0420 code before repair put in a bottle of cataclean and everything clear out. My gas light came on and few minutes later the check engine light came on with the p0420 code. the mechanic says it the upstream 02 sensor. any suggestion

    4. My 2013 Ford Focus SE. I changed the catalytic converter and upstream and downstream O2 sensors, and I’m still getting the code P0420. Any information on what can cause this? Any help would be grateful.

      1. This happened to me. I was within an inch of buying a new cat when I spotted an item on a noticeboard about using the wrong oil. The lazy sods at the garage had put 15w30 in at the last service. Dumped that andput 5w20 in plus a new filter and have been running the engine on 98 octane to clear the cat. Cleared the error with my 20 quid scanning tool and haven´t had a problem since. HTH

      2. i just replaced two 02 sensors and still failing emissions

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