Ok …here goes…fasten your seatbelts…
We are not to receive …if we are ‘
conscious’ of mortal sin.
usccb.org/liturgy/current/intercom.shtml
Canon Law on the matter…
Can. 916 A person who is conscious of grave sin is not to … receive the body of the Lord without previous sacramental confession unless there is a grave reason and there is no opportunity to confess; in this case the person is to remember the obligation to make an act of perfect contrition which includes the resolution of confessing as soon as possible.
(grave sin in the above …means mortal sin…for the terms grave sin, mortal sin and serious sin are used interchangeably …in this sense it means being conscious of committing the sin with full knowledge and deliberate consent)
(an example of the exception above is say a person is out in place where a priest does not come for months at a time and they have a grave reason…normally one needs to confess mortal sins…the fact that one intends to go to Confession but missed the chance as given in some examples above…would NOT be what this means)
Now to your question.
If one has honest doubt as to if they have committed a mortal sin then they by very definition are not
conscious of committing a mortal sin. So they may receive Holy Communion. (I would also make an act of perfect contrition)
Now they should of course if they think they may have committed a mortal sin…examine themselves well…(but without scruple).
(a person who is lax or around lax should give greater pause and consideration)
(also one need not go to communion at each Mass…and it can be good to abstain from time to time for the sake of those who cannot…but I am not speaking so much to those with scruples…in saying this.)
Compendium of the Catechism:
- What is required to receive Holy Communion?
1385-1389
1415
To receive Holy Communion one must be fully incorporated into the Catholic Church and be in the state of grace, that is, not conscious of being in mortal sin. Anyone who is conscious of having committed a grave sin must first receive the sacrament of Reconciliation before going to Communion. Also important for those receiving Holy Communion are a spirit of recollection and prayer, observance of the fast prescribed by the Church, and an appropriate disposition of the body (gestures and dress) as a sign of respect for Christ.
- When does one commit a mortal sin?
1855-1861
1874
One commits a mortal sin when there are simultaneously present: grave matter, full knowledge, and deliberate consent. This sin destroys charity in us, deprives us of sanctifying grace, and, if unrepented, leads us to the eternal death of hell. It can be forgiven in the ordinary way by means of the sacraments of Baptism and of Penance or Reconciliation.
From the Catechism:
1857 For a sin to be mortal, three conditions must together be met: "Mortal sin is sin whose object is grave matter and which is also committed with full knowledge and deliberate consent."131
1858 Grave matter is specified by the Ten Commandments, corresponding to the answer of Jesus to the rich young man: "Do not kill, Do not commit adultery, Do not steal, Do not bear false witness, Do not defraud, Honor your father and your mother."132 The gravity of sins is more or less great: murder is graver than theft. One must also take into account who is wronged: violence against parents is in itself graver than violence against a stranger.
1859 Mortal sin requires full knowledge and complete consent. It presupposes knowledge of the sinful character of the act, of its opposition to God’s law. It also implies a consent sufficiently deliberate to be a personal choice. Feigned ignorance and hardness of heart133 do not diminish, but rather increase, the voluntary character of a sin.
A person with scrupulosity is usually recommended to confess only “certain” sins not doubtful ones.
Now again…a person with scruples can have a difficulty in the matter of judging if they committed a mortal sin or not…they need to follow the advice of their confessor…and the principles given them … they too can ask themselves…are they really ‘conscious’ of committing a mortal sin???
For a person with a an ordinary conscience it can be recommended to confess doubtful mortal sins…
A person with a lax conscience is a different matter they are much more strongly recommended to confess doubtful mortal sins…for they would not be bothered so easily…and one classic spiritual writer (Tanquerey) even states that when it is a question of full consent…the presumption is that the lax person did consent otherwise he would not be bothered so much by it…(his opinion…but a good one…for someone who is really lax…is well LAX)
Also I would add that if one say does something…some action…that is grave…like watch pornography…or some lustful act…etc …but thinks they may not have had the full consent from some reason or another…but they actually did the thing…I would recommend they see their confessor first… in such a case…their confessor will be able hopefully also to assist them not only with absolution but with advice…(of course see one who is faithful to the Church…).