Catania, facing almost certain relegation one season after equalling their best-ever Serie A finish, immediately named Maurizio Pellegrino as Maran's replacement while Cagliari described the decision to part company with Lopez as "painful".
Maran, 50, who led Catania to eighth place last season when they just missed out on the Europa League, was sacked for the first time in October after a poor start to the season.
His replacement Luigi De Canio fared little better and was himself dismissed in mid-January with Maran returning to the hotseat.
However, Catania have taken only one point from their last eight games, leaving seven points adrift of safety and almost certain to be relegated after eight seasons in the top flight.
The final straw was Sunday's 2-1 home to defeat from Torino, when the Sicilians led from the second minute before conceding two goals in the final half hour.
Pellegrino has coached several clubs in the second and third tiers of Italian football and was in charge of Catania when they were a Serie B side in 2002.
Cagliari parted company with Lopez after losing 3-1 at home defeat to AS Roma on Sunday.
"The need to steer the team back on course and to safety at the end of the championship means that a change on the first team bench is indispensable," Cagliari said in a statement.
"The decision is all the more painful because of how much Lopez has devoted as a professional and a person today, and in the past as captain of the team."
Lopez, a former Uruguay defender who spent 12 years of his playing career at the club where he retired in 2010, had been promoted from assistant coach at the start of the season.
The Sardinians have lost four of their last five matches and dropped to 15th in the table with 32 points, seven clear of the relegation zone with six games to play.
There have been 14 changes so far this season in Serie A with three at Catania, two each at Cagliari and Sassuolo and one apiece at Genoa, Sampdoria, Lazio, Bologna, Chievo, AC Milan and Livorno.
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