A DOC provides the same function as the SCR system except the SCR can provide for two reactions that reduce or eliminate NOx while a DOC, avaliable in two different versions, can only provide for one or the other. If I were deleting the SCR and DPF, I would have a DOC installed. I don't think removing the EGR will make a big difference noise-wise and since all you have is the DPF and probably a combination DOC, I don't think you'll hear too much of a difference. I recommend using a DOC anyway.

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