Fainting invalidates wudoo’
Yes, the scholars are unanimously agreed that fainting is one of the things that invalidate wudoo’, even if it is brief.
Whoever faints and loses consciousness, even for a moment, has broken his wudoo’.
Ibn Qudaamah said in al-Mughni (1/234):
Loss of consciousness due to insanity, fainting, drunkenness and medicines that cause loss of consciousness invalidates wudoo’ whether it is brief or long, according to scholarly consensus. Ibn al-Mundhir said: The scholars are unanimously agreed that the one who has fainted must do wudoo’.
These people lose consciousness more deeply than does one who is asleep, based on the fact that they would not come around if someone tries to rouse them. The fact that wudoo’ is required of one who sleeps indicates that it is required in cases that are more extreme than that. End quote.
Al-Nawawi said in al-Majmoo’ (2/25)
The ummah is unanimously agreed that wudoo’ is invalidated by insanity and fainting. Consensus on this point was narrated by Ibn al-Mundhir and others, and our companions are unanimously agreed that whoever loses consciousness because of insanity, fainting, sickness, drunkenness, etc, or who takes medicine because of necessity or any other reason and loses consciousness, his wudoo’ is invalidated.
Our companions said: The drunkenness that invalidates wudoo’ is that in which a person loses consciousness, not the initial feelings of relaxation. And our companions said: there is no differentiation in all these cases between one who is sitting firmly on his backside and one who is not, or whether the amount is a little or a lot. End quote.
Shaykh Ibn ‘Uthaymeen was asked: Is wudoo’ invalidated by fainting?
He replied: Yes, wudoo’ is invalidated by fainting, because fainting is more extreme than sleep, and sleep invalidates wudoo’ if it is deep and the sleeper is unaware if anything comes out of him. As for light sleep in which the sleeper would realize if something comes out of him, this kind of sleep does not invalidate wudoo’, whether the person is lying down, or sitting and reclining, or sitting but not reclining, or in any other position. So long as he realizes if something comes out of him, this sleep does not invalidate wudoo’. But fainting is more extreme than sleep, so if a person faints, then he has to do wudoo’. End quote.
Majmoo’ Fataawa Ibn ‘Uthaymeen, 11/200
Shaykh Ibn Baaz (may Allaah have mercy on him) was asked: What is the ruling on the wudoo’ of those who experience moments of stupor?
That depends. If it is brief and the person does not actually lose consciousness and it does not prevent him feeling it if he passes wind, then it does not matter, such as one who feels drowsy but does not sleep deeply, rather he can hear movements. It this case it does not matter because he knows if something comes out of him. The same applies if the stupor does not cause lose of consciousness. But if the stupor prevents him from feeling if something comes out of him, as in the case of drunkenness or one who has a sickness that causes him to lose consciousness to the extent of stupor, then this invalidates his wudoo’, as in the case of fainting and epilepsy. End quote.
Fataawa al-Shaykh Ibn Baaz, 10/145.