OBD2 Codes

Ford P0420 – Meaning, Causes, Symptoms, & Fixes

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


Ford Code P0420 Definition

Catalyst system efficiency below threshold (Bank 1).

What Does P0420 Mean For Your Ford?

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

Ford P0420 Symptoms

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

Ford P0420 Causes

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

Ford 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 Ford 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.

Ford Code P0420 Common Diagnosis Mistakes

Do not overlook other codes that are paired along with P0420. Ford 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.

Ford P0420 Diagnosis

Tools Needed to Diagnose:
  1.  Scan your Ford 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 Ford 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.


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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

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