A P0420 diagnostic trouble code (DTC) is a very common trouble code that's going to illuminate the check engine light on your Ford Fiesta sooner or later.
In this tutorial, I'm going to explain what the P0420 trouble code means, what causes it to pop up, some of the most common causes of the trouble code and how you can resolve it.
Contents of this tutorial:
APPLIES TO: This tutorial applies to the following vehicles:
- 1.6L Ford Fiesta: 2011, 2012, 2013, 2014, 2015, 2016, 2017, 2018, 2019.
What Does Trouble Code P0420 Mean?
Trouble code P0420: Catalyst System Efficiency Below Threshold indicates an issue with the efficiency of the catalytic converter in your Ford Fiesta's exhaust system.
In a nutshell, the catalytic converter's primary role is to reduce the emission of harmful pollutants by converting them into less harmful gases through a chemical reaction.
As the engine runs, the fuel injection computer monitors the converter's efficiency by comparing the readings from the oxygen sensors located before and after it.
If the computer finds that the converter is not adequately reducing the levels of hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx) in the exhaust, it will trigger the P0420 code.
On your Ford Fiesta, the catalytic converter is located on the exhaust pipe right after the exhaust manifold; as a matter of fact, it's part of the exhaust manifold assembly, as the photo at the beginning of this article shows.
Common Symptoms Of A P0420 Trouble Code
For the most part, when the catalytic converter fails and the fuel injection computer illuminates the check engine light with a P0420 trouble code, you're not going to really notice any engine performance problems.
Although this is not the most exhaustive list of symptoms, you're going to see one or more of the following issues:
- The check engine light is on (triggered by the P0420 DTC).
- Bad gas mileage.
- Rotten egg or sulfur smell coming out of the tailpipe when the engine is running (especially when you accelerate the engine under load).
In some rare cases, the catalytic converter fails and breaks into pieces. When this happens, you'll notice a lack of power when you accelerate the engine under load as the broken pieces block the exhaust from exiting the catalytic converter.
What Causes A P0420 Trouble Code?
As mentioned earlier, the cause of the P0420 trouble code is a failing catalytic converter.
One thing to keep in mind is that a bad oxygen sensor will not cause the fuel injection computer to set a P0420 trouble code, although a failing O2 sensor can contribute to the failure of the catalytic converter.
It's been my experience that the catalytic converter will fail for one of the three following reasons:
- Normal Wear And Tear: The catalytic converter's filtering element simply wears out.
- Rich Air/Fuel Mixture Problem: A fuel injection issue is causing the engine to run rich. This rich condition overloads the catalytic converter, causing it to overheat. This excessive amount of heat causes the converter's filtering element to fail.
An excessively rich air/fuel mixture is usually caused by one of the following problems:- Ignition system misfire.
- Leaking fuel injector(s).
- EVAP canister leaking fuel into the intake manifold.
- Engine Burning Oil: The engine is burning oil and the catalytic converter's filtering element gets covered in soot and carbon from the burning oil.
- Engine Coolant Contamination: The engine has suffered a blown head gasket and coolant has entered into the exhaust and reacted with the catalytic converter's filtering element.
How To Resolve A P0420 Trouble Code
If a trouble code P0420 is illuminating the check engine light, you'll need to replace the catalytic converter.
Once the catalytic converter's filtering element has simply worn out or has become contaminated (with engine oil or engine coolant), there isn't much that you can do to make it work efficiently again. Replacing it is the only option in about 99% of the cases.
In the next section, I'll go into some of the things that you need to keep in mind before you replace the catalytic converter.
Before You Replace The Catalytic Converter
Before you replace the catalytic converter, it's important to check for any underlying causes that may have caused it to fail.
As I mentioned earlier, the catalytic converter can fail on its own; even the most well-maintained, issue-free Ford Fiesta will eventually experience catalytic converter failure simply due to normal wear and tear.
If your Ford Fiesta is currently experiencing any issues (besides the P0420 diagnostic trouble code), such as a cylinder misfire or a problem with the fuel injection system that is triggering a different trouble code, you'll need to address and resolve them first before replacing the converter.
Otherwise, those unresolved engine performance issues will cause the new converter to fail within a matter of weeks or months.
I recommend that you troubleshoot and resolve any problem that could be causing:
- An excessive rich air/fuel mixture that could be overloading the exhaust with hydrocarbons (raw gasoline).
- Engine coolant (anti-freeze) to enter the exhaust gas.
- Engine oil to enter and burn in the engine cylinders.
In my experience, the most common cause of a bad catalytic converter has been a misfire problem. So if you have a misfire trouble code stored in the fuel injection computer's memory (P0300, P0301, P0302, P0303, P0304), solve it first.
If any other trouble codes are present along with the P0420 trouble code, troubleshoot and resolve them first before replacing the catalytic converter.
More 1.6L Ford Fiesta Test Tutorials
If this tutorial was helpful, be sure and take a look at all of the 1.6L Ford Fiesta diagnostic tutorials in this index:
Here's a sample of the tutorials you'll find there:
- How To Test The MAF Sensor (2011-2013 1.6L Ford Fiesta).
- How To Test Engine Compression (2011-2019 1.6L Ford Fiesta).
- How To Test For A Blown Head Gasket (2011-2019 1.6L Ford Fiesta).
- Fuel Injector Circuit Wiring Diagram (2011-2013 1.6L Ford Fiesta).
If this info saved the day, buy me a beer!
