Here are the general guidelines regarding ability checks:
The GM calls for an ability check when a character or monster attempts an action (other than an attack) that has a chance of failure. When the outcome is uncertain, the dice determine the results.
In some cases, you could want to fail an ability check. If you are climbing a wall an want to drop off, you fail simply because you stop attempting to climb, but in other cases, success and failure are both tied to the actual action and may both have desirable outcomes.
Example 1: Spell scrolls
An evil adversary wants you to cast a spell scroll for him, but you have him figured out and want to destroy his spell scroll instead, seemingly by accident. The rules on spell scrolls read:
If the spell is on your class’s spell list but of a higher level than you can normally cast, you must make an ability check using your spellcasting ability to determine whether you cast it successfully. The DC equals 10 + the spell’s level. On a failed check, the spell disappears from the scroll with no other effect.
A failure means that you successfully destroy the spell scroll.
Example 2: Amulet of the Planes
You are in possesion of the Amulet of the Planes. Your party is being overrun by terrible monsters, and the only way to save your party is to sacrifice yourself by sending yourself and the monsters to another plane of existence.
While wearing this Amulet, you can use an action to name a location that you are familiar with on another plane of existence. Then make a DC 15 Intelligence check. On a successful check, you cast the Plane Shift spell. On a failure, you and each creature and object within 15 feet of you travel to a random destination. Roll a d100. On a 1-60, you travel to a random location on the plane you named. On a 61-100, you travel to a randomly determined plane of existence.
By walking up to the monsters and failing the check, you and the monsters would be sent to another plane of existence.
The question
The rules seem to assume that you always want to succeed when you perform an ability check. Sometimes, however, you could want to fail on purpose. A sensible DM would rule that if you are skilled enough to succeed the check, then you would undoubtly be able to fail it on purpose as well. What does the rules as written say — if anything — about this circumstance?
According to RAW, is it possible to attempt an ability check and intentionally fail?
Best Answer
Only if you fail a dice roll
A character cannot "fail an ability check", because characters don't make ability checks. Ability checks are nothing but game mechanics, they are basically dice rolls. Characters take actions instead — these actions, not dice rolls, are the subject to be succeed or failed.
PHB page 6 "How to play" describes the basic gaming pattern:
The DM might (but don't have to) ask the player for an ability check between steps 2 and 3. They might just say "yes, you did it" without any rolls. That depends on the action the character does (the very action the player announced at step 2).
At step 2 you describe your character's intent — the DC for an ability check will be based on this intent. If you announce "I'm trying to activate a spell scroll" it is one particular intent. If you say "I'm trying to destroy a spell scroll by activating it incorrectly" it is another intent. DM might ask for an ability check in the second case, or just allow it automatically — it isn't specified in the rules, so is completely up to the DM. The same for the improper use of Amulet of the Planes.