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A engine checklght appeared on my 2004 Hyundai Elantra GT hatchback, and was diagnosed as a faulty catalytic converter, that was considered normal for the age of the car. About 3 days later when i started the car in the morning, the engine was very loud but the checklight had disappeared. I thought the loudness of the engine was an issue with the exhaust (maybe a hole) and that for some reason it removed the checklight. I did not drive the car for about 3 days. Today I wanted to drive it to my mechanic. The car drove normally, without a loud noise and no checklight. Should I be concerned?

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Not knowing the reason for the loud noise would disconcerting to me. Usually load noises from engine or any other system is a indication of a serious problem. I would definitely have a mechanic check it out. A catalytic check engine light to me would indicate the downstream O2 sensor detected a defective cat. This followed by a lot of noise then a absence of the check engine light afterwards could mean damage to that O2 sensor.

I think a new cat and both O2 sensor need to replaced and all the computer codes checked to find out why the cat failed in the first place otherwise the problem will reoccur.

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The catalytic converter is actually a set of thin "tubes" like a square grid that has been extruded. This tube-like system improves the surface of the converter that comes in contact with the gases. With time, this "grid" crunches in a way that produces long "needle" like fragments. These fragments very often stuck in the grids holes preventing the air been exhausted properly, thus reducing power and even changing the sound of exhaust system. You should have your converter examined for such a case because this case, if worsens could lead to engine issues such as overheating, stalling, inability to start etc.

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