Learn Spanish (Latin America): Quizás la primera marcha no entra por el frío. - Maybe the first gear doesn't engage because of the cold.

Words like quizás can take either the indicative or the subjunctive depending on how certain the speaker is. Using the indicative (entra) shows that the speaker feels this is a very likely or probable explanation. If they were making a wild guess or felt more doubtful, they would use the subjunctive (entre).

Why is the verb entrar used for a car gear engaging?

In Spanish, it is standard to use entrar (literally "to enter") when talking about mechanical parts fitting together or gears shifting. Saying la marcha no entra is the most natural way to express that "the gear won't go in" or "doesn't engage."

Could I say para el frío instead of por el frío?

No, because por is used here to show the cause or reason for an action. The phrase por el frío translates to "because of the cold." If you used para, it would mean "for the purpose of the cold," which doesn't make sense in this context.

Is there any difference between using quizás and quizá in this sentence?

Both forms mean "maybe" and are practically interchangeable. Speakers often choose quizás before a word starting with a vowel to make pronunciation flow better, but using it before a consonant like la, as seen here, is completely fine and very common in everyday speech.