Replacing whichever parts are generally involved in failure is plain common sense.

CN90 - your example of the cooling system systemic issue on the E39 is an excellent one. Where parts within a system are known to fail, only a sub0standard technician would replace only the failed part in that system. The reason being, they are generally asked to warrant their work, and how can that occur when they know there are other failure-prone elements they have not at the least inspected, at best replaced.

Do it once, do it right.

To misquote an old adage - a rich man fixes something once. A poor man fixes something many times.

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Cheers,

Anthony