Well, it's important to remember that the Iron Guard wasn't monolithic even prior to Codreanu's death; there were factions within the Iron Guard even during Codreanu's lifetime. There was Codreanu's faction, which was the largest and most powerful and dictated the mainstream of the Guard's ideology; Horia Sima's faction, which was (initially) more pragmatic politically and less fanatical religiously; and the faction led by Ion Moța and Vasile Marin, which was the most religiously fanatical. Moța and Marin had been killed in 1937 while fighting on the Francoist side of the Spanish Civil War, almost two years before Codreanu was assassinated, but their ideas still held some currency within the Guard.

Because Codreanu was such a charismatic figure and his personality and leadership were crucial for the development of the Legionary movement, his death created a power vacuum at the top of the party and led to a power struggle between Sima, the most prominent surviving leader, and Vasile Noveanu, another prominent figure, who detested Sima. Noveanu had been living in exile prior to 1939 and had created a core group of Legionaries around himself, in opposition to Sima's faction. Sima had been unpopular with many members of Codreanu's majority faction (including Codreanu's brother and father) due to his less radical ideology, and this mutual enmity with Sima and his faction facilitated Noveanu's bid for power within the party; he had mostly consolidated his power by early 1940.

Sima and many of his close associates fled to Germany in early 1939 in the wake of Codreanu's assassination, suspecting that they might be next in line. Sima expected the Nazi leadership to support him in a planned coup against the existing government of King Carol II due to the ideological alignment of Legionary and Nazi ideology, but that ultimately didn't happen. Noveanu, meanwhile, took a more politically pragmatic line upon his return to Romania and decided that the best way to secure the Iron Guard's survival was to ally with Carol, rather than continuing its conflict with his regime, which had become increasingly dictatorial from 1938 onward. Sima's group returned to Romania in mid-1939 and later carried out the assassination of Prime Minister Armand Călinescu on 21 September. Sima was briefly involved in two of the subsequent governments, but didn't achieve political power in any sustained way, while Noveanu's faction remained loyal to Carol and his regime, and convinced Carol to issue a blanket amnesty for Iron Guard members in April 1940.

It was this allegiance to Carol II that eventually proved to be the Noveanu faction's downfall. After Romania lost Bessarabia and Bukovina to the Soviet Union and Northern Transylvania to Hungary, Carol II's regime lost public support and he was forced to abdicate, in part due to the threat of a coup by Sima's faction of the Iron Guard; his abdication also undermined Noveanu's position within the Iron Guard, and Sima was able to force him out of power in September 1940, shortly before Sima and the Iron Guard entered into government through their alliance with the new leader, Ion Antonescu. Of course, Sima's time in power was also short-lived; after the Iron Guard attempted a coup against Antonescu in January 1941, which was repressed by the military (which remained loyal to Antonescu), Sima was forced to flee to Germany (and later to Spain). Noveanu actually remained in Romania, unlike many Legionaries; he was placed under surveillance and imprisoned at least once by the Communist regime.

The divide between the Codreanu and Sima factions is still present in the modern neo-Legionary movement, both claiming the to be the legitimate followers of Codreanu's original teachings. It's worth noting that these factions are very small, fragmentary groups within the larger far-right movements, such as the New Right (Noua Dreaptă).

Sources:

Jean Ancel, The History of the Holocaust in Romania (Yad Vashem, 2011)

Roland Clark, Holy Legionary Youth: Fascist Activism in Interwar Romania (Cornell UP, 2015)

Nicholas M. Nagy-Talavera, The Green Shirts and the Others: A History of Fascism in Hungary and Romania (Hoover Institution, 1970)

Zigu Ornea, Anii treizeci: Extrema dreaptă românească (FCR, 1995)