Hacer falta v.s necesitar - Explanation needed
I wrote
¿Cuántos grados hace falta el horno para cocinar el pastel? - How many degrees does the oven need to cook the cake?
But this was suggested
"Hace falta" doesn't have a subject, but you have one in your sentence, el horno, so this idiomatic expression doesn't fit here. Use necesita instead.
It should be
¿Cuántos grados necesita el horno para cocinar el pastel? -
Can someone explain me the correction?
8 Answers
Hey that was my suggestion! 
I''l give you two examples on how to answer the question you gave (Pardon if the translation is a bit faulty.)
1) El horno necesita 300 grados para cocinar el pastel. (The oven needs 300 degrees to bake the cake.)
2) Hacen falta 300 grados para cocinar el pastel. (300 degrees is needed to bake the cake.)
In the 1st answer we are saying outright that it is the oven that needs 300 degrees for the cake to be baked.
In the 2nd answer using the idiomatic expression hacer falta (specifically, hacen falta since grados is plural), we are expressing the need in general to heat the oven to 300 degrees in order to bake the cake. (Of course, we imply that the cake needs an oven to bake in.)
Another example:
Los estudiantes necesitan $4000 para matricularse en la universidad.
Hacen halta $4000 para matricularse en la universidad.
In the 1st one we are saying that the students specifically need $4000 to enroll in the university. In the 2nd one, we are talking about a general need for or requirement of $4000 to enroll, that is, without mentioning anyone in particular.
Edit: After a closer, more "intense" analysis, I realized that I made a booboo. There is a subject for "Hacer falta." But the subject is not whatever it is that needs something, it is the thing (or things) that is/are needed or lacking. That explains why it agrees with the subject in number. So my suggestion stands as it was (that you need necesitar instead of hacer falta in your sentence), but please just disregard what I said that hacer falta doesn't have a subject. I'm terribly sorry about that, dew.
All right, I am posting this answer by request. Here goes my interpretation of the whole mess. Let's start with the source of the problem:
¿Cuántos grados hace falta el horno para cocinar el pastel? - How many degrees does the oven need to cook the cake?
It should be
¿Cuántos grados necesita el horno para cocinar el pastel? -
First of all, in my very humble opinion, that is a very strange way to pose a simple question such as "At what temperature should I bake the cake?" But anyway, fine - we want to ask specifically how many degrees must be reached within the oven in order to bake a cake (seriously? Don't you find this is weird? - must be me, then).
The problem with the original attempt is that it is not correctly written. You could say, "¿cuántos grados hacen falta para que el horno pueda cocinar el pastel?" This means "how many degrees are required in order for the oven to cook the cake?"
Samdie is right in that "cuántos grados le hacen falta al horno para cocinar..." should be strictly interpreted as "how many degrees is the oven lacking...", although on first reading I'd probably go ahead and interpret it as originally meant, i.e.: how hot does do oven need to be?
I agree that the corrected version: "¿Cuántos grados necesita el horno para cocinar el pastel? " is probably the best way to express such an awkward expression. More direct and natural, no doubt.
But I understand that the issue is: how can I use "hacer falta"?
"Hacer falta" means to miss something, to need someting: "¿Qué hace falta para que podamos cocinar el pastel?" - What's needed in order to cook the cake? (What is lacking that we can provide in order to accomplish this? - temperature).
"¿No te hace falta tu dosis diaria de SD?" Don't you miss your daily dose of SD?
"Me hace falta dormir más" - "I need to sleep more (I don't have enough of this, I am lacking sufficient sleep).
To close, as a native speaker, it simply would not occur to me to state the question in those terms. Not that it can't be done, it just doesn't strike me as natural, is all.
With all due apologies to those who prefer commonly used expressions over mathematically/scientifically accurate expressions, "¿Cuántos grados hacen falta el horno para cocinar el pastel?" Does not ask "What should the temperature be?" but, rather, "How many degrees are lacking?" If the oven is set at 300 and the temperature should be at 450, then 150 degrees are lacking (faltan 150 grados).
The thing that is lacking something is the indirect object, what it is lacking is the direct object:
La temperatura del horno le hace falta 100 grados.
Le hace falta 100 grados la temperatura del horno.
¿Cuántos grados necesita el horno para cocinar el pastel?
Dewclaw ,your statement is correct, I called a friend to confirm it.
Hace falta cocinar el pastel a 300 grados por media hora.
Se necesita cocinar o que se cocine el pastel a 300 grados por media hora.
That's the maximum you can bake that cake (heat wise) 
To say it is an idiomatic expression in Spanish means No, it cannot be explained in English. Hace frío, hace calor I'm sure you know. Tú me haces falta, I miss you, how could that be explained?
Wikipedia:
Idiom (Latin: idioma, “special property”, f. Greek: ?????? — idi?ma, “special feature, special phrasing”, f. Greek: ????? — idios, “one’s own”) is an expression, word, or phrase that has a figurative meaning that is comprehended in regard to a common use of that expression that is separate from the literal meaning or definition of the words of which it is made.
In linguistics, idioms are usually presumed to be figures of speech contradicting the principle of compositionality; yet the matter remains debated. John Saeed defines an “idiom” as words collocated that became affixed to each other until metamorphosing into a fossilised term.[3] This collocation — words commonly used in a group — redefines each component word in the word-group and becomes an idiomatic expression. The words develop a specialized meaning as an entity, as an idiom. Moreover, an idiom is an expression, word, or phrase whose sense means something different from what the words literally imply. When a speaker uses an idiom, the listener might mistake its actual meaning, if he or she has not heard this figure of speech before.[4] Idioms usually do not translate well; in some cases, when an idiom is translated into another language, either its meaning is changed or it is meaningless.
