If consensus has been reached, it is not permissible for anyone differ from it
If consensus has been reached, it becomes binding shar‘i proof, and what is required is to follow it; it is not permissible to differ from it.
Imam ash-Shaafa‘i (may Allah have mercy on him) said: The command of the Messenger of Allah (blessings and peace of Allah be upon him) to adhere to the main body of the Muslims is one of the proofs that the consensus of the Muslims – in sha Allah – is binding. End quote.
Ar-Risaalah, p. 403
As-Sarkhasi (may Allah have mercy on him) said:
Consensus is definitive proof in the same way as texts (of Qur’an and Sunnah). Just as it is not permissible to overlook a shar‘i text because of some view that is contrary to these texts, it is also not permissible to go against consensus because of a different view that came along after consensus was formed on the basis of sound evidence. End quote.
Usool as-Sarkhasi, 1/308
Shaykh Muhammad ibn ‘Uthaymeen (may Allah have mercy on him) said, in the commentary on his book al-Usool min ‘Ilm al-Usool (p. 498):
If the Sahaabah were all agreed on any issue, then their consensus disallows any differing view from being valid. Hence it is not permissible for anyone who comes after them to differ from their consensus, because consensus disallows any different views, as it constitutes binding proof. End quote.
Going against consensus may reach the level of kufr, if consensus was definitely reached and there is no doubt concerning it.
Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) said:
Everything on which they are agreed must have been based on a text from the Messenger. Every issue on which there was definitely consensus, and there are no different views among the believers, is guidance from Allah, and going against such consensus constitutes kufr, just as it is kufr to go against a clear text. However, if it is thought that there was consensus, but it is not definite, then in this case we cannot be certain that this is guidance on the basis of the teachings of the Messenger. The one who differs from consensus in such cases is not regarded as a kaafir; rather the idea that there was consensus may be mistaken, and the correct view may be something other than that. This is the decisive answer concerning who may or may not become a kaafir by differing from consensus. End quote.
Majmoo‘ al-Fataawa, 7/39
Thus it is clear that once it is proven that there was consensus, it becomes binding shar‘i proof, binding upon all Muslims, and it is not permissible for anyone to differ from it on the basis of “making ijtihaad” or any other claim.
And Allah knows best.