Is the Catechism of the Catholic Church considered scripture?

According to Catholicism, the Catechism has the same authority as the Bible, its existence is borne out of the same body of folks who brought you the Bible as a compilation of revealed truth in the first place.

However, it can't be conisidered a substitute for the Bible and no, it's not scripture any more than the Evangelical Councils were scripture. It is the the synthesis of Magesterial Teaching, tradition and scripture. It owes its existence to synods of Bishops following the Second Vatican Council placing an emphasis on general Catechesis.

Any Catholic will tell you it belongs right next to your Bible because there is no book which gives so lucid an explanation of the faith. But, it didn't exist until the 1990's so... The more fundamental question is, does preaching against the doctrines of the Catholic Church make you a heretic?

The Catechism is a 'sure norm for teaching the faith' and a 'valid instrument for ecclesial communion'. It's used for teaching directly, and for adapting down to particular catechisms (YouCat, Adult Catechism, Rite of Christian Initiation for Adults (RCIA), etc...). So, one caveat in the Catechism is that it is catholic (i.e. universal) and it does not adapt the doctrine adequately to particular cultures, ages, and spiritual states of the reader and as such there may be reasons for emphasizing certain aspects of authentic Catholic Doctrine that may at first seem at odds with the Catechism, I think this happened recently with the YouCat, which was geared towards the under 30 crowd and had more concrete answers to questions about sex and money that you'd get by reading the Catechism. Therefore, you wouldn't automatically be a heretic if you preach something that only seems at odds with the Catechism, it really has to be at odds with the totality of Catholic Doctine. If you preached for birth control or abortion or same sex "marriage" or euthanasia or divorce you would be on your way I becoming a heretic, but you'd have to persist in your heresy to truly be a heretic. If you completely give up the faith, then you're an apostate and if you preach something that just isn't very Catholic, like "the Pope isn't very smart" then you're a schismatic. But, if you were never a Catholic to begin with, then you can't be any of these things.

Finally, because I just read it and I don't know whether you want to know this, but the General Directory for Catechesis says:

the catechism of the catholic church is not the only source of Catechesis, since as an act of the Magesterium, "it is not superior to the word of God but at its service." However it is a particularly authentic act of interpretation of that word, such that the Gospel may be proclaimed and transmitted in all it's truth and purity.

The immediate answer is “No, it is not scripture.” Scripture is scripture and adding to it or subtracting from it is a no-no.

As to “Does it have the same authority as the Bible?” Well, I suppose the answer is “authority for what?” For some things the authority of the Catechism is superior (certain areas of theology are only hinted at in the Bible). It can be safely said, though, that there is nothing in the Catechism which contradicts the Bible (if the Catechism is taken as a whole and in light of a Catholic understanding of Scripture).

Can you contradict the Catechism without being called a heretic? That depends on how you contradict the Catechism. On certain matters it is more definitive than others and on some matters it is 100% correct. On other matters it is a bit more theologically ambiguous. It is important to remember:

This catechism aims at presenting an organic synthesis of the essential and fundamental contents of Catholic doctrine, as regards both faith and morals, in the light of the Second Vatican Council and the whole of the Church’s Tradition. Its principal sources are the Sacred Scriptures, the Fathers of the Church, the liturgy, and the Church’s Magisterium. It is intended to serve “as a point of reference for the catechisms or compendia that are composed in the various countries”. (CCC 11)

That word aims is important, because it is not a perfect synthesis and there are many, many finer points which can be debated and there is quite a bit of room for disagreement (you are talking about a Church which has a 1400 year old debate with its neighbor over one word). And it even testifies the the fact that it has not gotten everything perfect for all times and all people:

By design, this Catechism does not set out to provide the adaptation of doctrinal presentations and catechetical methods required by the differences of culture, age, spiritual maturity, and social and ecclesial condition among all those to whom it is addressed. Such indispensable adaptations are the responsibility of particular catechisms and, even more, of those who instruct the faithful:

Whoever teaches must become “all things to all men” (⇒ I Cor 9:22), to win everyone to Christ. . . Above all, teachers must not imagine that a single kind of soul has been entrusted to them, and that consequently it is lawful to teach and form equally all the faithful in true piety with one and the same method! Let them realize that some are in Christ as newborn babes, others as adolescents, and still others as adults in full command of their powers.... Those who are called to the ministry of preaching must suit their words to the maturity and understanding of their hearers, as they hand on the teaching of the mysteries of faith and the rules of moral conduct. (CCC 24)

All of that said, it is very difficult to contradict the Catechism without contradicting some important and definitive doctrine.

I know of one place, for example, which is phrased very poorly (in an attempt to guide pastors to a more compassionate action). My disagreement is perfectly acceptable and reasonable (as I agree with the teaching but not the admonition which goes along with it). On the other hand, if I were to disagree with the teaching behind the phraseological error I actually would be contradicting some very authoritative doctrines in the Church. That would be, at a minimum, a form of heresy.