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.