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Hi
Please tell me if in them is necessary here.
Silencer pipes should be closely examined to find out if there are any blocks in them.
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Although awkward, it's an acceptable sentence assuming that your interlocutor knows what a block refers to.
A more natural rephrasing might be:
Silencer pipes should be closely examined for blocks (or, blockage).
Comments  
Thank you, JohnParis.

Suresh