If consensus has been reached, it is not permissible for anyone differ from it
Question

                Does the ijtihaad of modern scholars override the consensus of the Sahaabah?
Praise be to Allah.
    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.