Ford P0420 – Meaning, Causes, Symptoms, & Fixes

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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Comments 1

Jimmie Stillwell

June 18, 2019

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.

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