Hello im new at this, i got a 2012 Mazda2 with s loud metal rattle at random times but mostly in when the car is cold. I was told i needed an 02 sensor and an exhaust manifold. Has anyone done this before, what tools will i need. Any help is appreciated.
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May or may not be helpful but..

I've changed the top O2 sensor, it was basically welded into the top of the manifold and when it eventually came out, all the threads were gone in the manifold. Had to have it repaired at a local garage.

I also chose to purchase a used sensor off ebay, which still works fine. I think new sensor alone is about £120 iirc.