The Check Engine light is on. It indicates that the catalytic converter needs replacing.
When a catalytic converter begins to fail, the computer will give a reading of the code P0420 and/or code P0430 which pertains to catalyst efficiency. The catalytic converter is, in a way, like a metal filter that traps toxic gases from leaving the combustion chamber, preventing them from entering the atmosphere. This is monitored by your Ford’s computer with the up and downstream oxygen sensors. If these oxygen sensors detect an error of some sort, a code will pop up in the computer. However, this does not always mean that the converters are failing. A worn out ignition system, faulty oxygen sensors and even bad gas can duplicate these codes as well. To be 100% certain that the catalytic converter is the problem, I would recommend enlisting the assistance of a certified mechanic to come by and inspect your Ford Expedition. They will have the experience and tools needed to determine if the problem lies with the catalytic converter or if the problem lies elsewhere. Once the cause is identified, they will be able to assist you in replacing your catalytic converter if necessary.
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