People in Maharashtra and Andhra celebrate sathyanarayana puja on full moon day. Puja will be made and a story will be told for 45 minutes. Even I did twice in Shirdi Sai Sathya Vratha. Is Lord satyanarayana avatar of Vishnu and mentioned in puranas?
No, it isn't because it's a late invention in Hinduism based on the Satya Pir Cult of 16th century Bengal.
"Satya Pir" over time became "Satya Nārāyaṇa" and came to be associated with Viṣṇu. So, any references to the Satya Nārāyaṇa pūjā in the Purāṇas are most likely late additions.
From The Islamic Syncretistic Tradition in Bengal by Asim Roy:
Satya-pir added a whole new dimension to the syncretistic process of pirification. Pirification, in this particular instance, seemed to relate more appropriately to the phenomenon of sanskritization, which applied to the Hindu society, than to Islamization. On close scrutiny the Satya-pir myth and cult emerges as a brāhmanical device to absorb the increasingly popular pir cult. The popularity of the cult seemed to have been greater among Hindus than Muslims, for the large majority of writings on the pir were Hindu contributions. As was the case with other popular pirs, the identity of Satya-pir seemed rather obscure, indeed even more than others. Although a few shrines bearing the name could be found, an examination of the relevant literature and tradition on the pir categorically rejects the historicity of the pir. Some traditions quite arbitrarily linked Husain Shāh, a fifteenth-century sultān of Bengal, with the introduction of this cult. Other commentators on the subject idealized it in general terms as a symbol of Hindu-Muslim syncretism. In the absence of hard historical evidence we attempt to offer a possible explanation of its origin on the basis of an internal analysis of its literature.
The literary tradition on the pir adopted either of two motifs, the brāhman or the merchant motif, both aiming at vindication of the pir in the same way as the Hindu mangalkāvya literature proclaimed the glory Of particular popular divinities. The first motif involved a brāhman who was advised by God to worship Satya-pir, appearing in the guise of a Muslim mendicant. The brāhman's refusal to accept a Muslim divine led to God's reappearance in the form of Kriṣṇa—a feat which finally convinced the brāhman. The second motif concerned a Hindu sea-merchant and his son-in-law, who, despite their initial scant regard for Satya-pir, were able to save their entire fleet from a storm, thanks to the steadfast devotion of the merchant's daughter to Satya-pir and the final submission of the whole family to the pir. Both these versions may be interpreted as underlining a process of upper-class Hindu acceptance of the Satya-pir cult. The process was closely paralleled by the gradual recognition of popular Hindu deities by the hieratic brāhmanical order. The growing popularity of pir and the pir cult in Bengal was at once a challenge and an opportunity for the brāhmanical priestly class. The brāhman answer, consistent with their past tradition, was to create a cult of pir in abstraction in Satya (eternity/truth)-pir and to establish an essential identity of and a continuity between old and new beliefs, with a view to its final absorption. It was not without significance that there were more Hindu writers on the Satya-pir tradition than Muslim, that God appeared before the brāhman in both Hindu and Muslim guises, and, finally, that there was also a clear attempt to divest the cult of its Muslim associations, Vidyāpati, a brāhman writer, called Satya-pir God's incarnation in the present age (kali-yug) "in a faqir's guise.' An old Sanskritic text, the Skanda-purān, was interpolated, and Satya-nārāyan emerged to dislodge Satya-pir. The brāhmanical acceptance of the pir cult was not easily achieved. The orthodox brāhmanical reaction was positively hostile to it, as evidenced by the life of Kanka, the author of Satya-pirer-kathā. A brāhman of Mymensingh, Kanka was allegedly brought up by an outcaste chandāl couple, both of whom died while he was a boy of five. He was recovered by a kindly and scholarly brāhman, Garga, in whose care he grew up into an intelligent young man, well versed in all learning. He earned a good reputation as a poet, and was later attracted to a pir, becoming his disciple. On the pit's instruction Kanka wrote a ballad on Satya-pir, which was "well received by both Hindu and Muslims devoted to Satya-pir." But when Garga proposed to restore Kanka to his brāhman caste, in a gathering of the brāhmans at his place of birth, the orthodox brāhmans led by Nandu vehemently opposed, on the ground of his chandāl upbringing and also for his receiving spiritual instruction from a Muslim. They accused Kanka of being a Muslim, and copies of his popular work on Satya-pir were "torn" and "burnt."
Despite what may, perhaps, be taken as the initial opposition of the Hindu orthodoxy, the syncretistic objects of the Hindu authors of Satya-pir tradition were clearly achieved. Their attempts did not, however, meet with success, if they intended total absorption of Satya-pir into Satya-nārāyan. Satya-pir and Satya-nārāyan remained with the masses of believers as one and the same. One Muslim writer combined them both into one "Satya-pir-nārāyan" or "Ghāzi Satya-nārāyan." Another found "Pir-nārāyan" sitting in Mecca. A third addressed Satya-pir as Ghāzi and added: "Thou art Brahmā, Viṣṇu, and Nārāyan."
In Sacred Tales of India, Dwijendra Nath Neogi offers a different theory:
Our own idea of Satya Pir pujāh is that, under the persecution which the Hindus suffered at the hands of the Islamites in India, the Brāhmans and their folk carried on their own religious rites under colours slightly false, and in this way Satya-Nārāyana came to be called Satya Pir. The rites of worship are not many. These, again, are not at all attractive. Simple in their character, they may be gone through within the privacy of the Hindu home. The name, therefore, was in such a case everything, the object being to delude Mussulmans into a belief that it was no Hindu deity, but a Mussulman saint who was being worshipped. No image, be it noted, is made of Satya Pir. The low class Mussulmans also worship him. They, however, set up an emblem which consists of an umbrella rolled up and a brazen stick with a circular plate at its top. Both the rolled-up umbrella and the stick are planted in the earth.
The other theory is that low class Hindus began to actually worship Mussulman pirs or local saints. This went on for some time, till the Brāhmans slightly changed the name of Satya Pir into Satya-Nārāyan. This was easy, considering that no image was set up of the deity worshipped. The great Chaitanya, on the other hand, was for some time worshipped by many ignorant Mussulmans under the name of "Pir Gorāchānd." Gorāchānd is a name of Chaitanya. His sanctity entitled him to the rank of a pir in the estimation of Mussulmans.
To the first question of whether Lord Satyanarayana is an 'avataar' of Lord Vishnu, the answer is no. To the second question of whether the same is mentioned in the puranas, the answer is also no, he is not mentioned in any of the scriptures. This vrataam has started around 100 or 200 years ago. The chapter that gets mentioned in the story is not there in 'Skanda Purnam' and moreover god is not a business person to hold grudges if something is not done properly or with mistakes.
Even now there are many traditional families who doesn't follow this ritual.