The Check Engine light is not on. However, when I took my car in for an inspection, the inspector said I needed to replace the catalytic converter/exhaust manifold because he said my vehicle made a noise when I pressed the accelerator. I don't see any drop in fuel mileage and do not detect any weird smells. It only makes the noise when I accelerate, and doesn't make any when going regular speed.
The catalyst inside of the converter can break away and cause rattling noises in the exhaust, especially on acceleration. Over time the catalyst material can break apart and cause partial plugging of the exhaust. If the catalyst is no longer able to convert hydrocarbons, then it is entirely possible that a fault for catalytic converter efficiency will be stored. Because the catalytic converter is an emissions device, you may be required to have it repaired even it if is giving you no issues depending on your local statutes. You may want to enlist the help of a mechanic, such as one from YourMechanic, who will be able to inspect the exhaust noise and perform the necessary repairs.
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