Method for manufacturing containers by blow moulding preforms made of thermoplastic, which comprises: a cyclic phase of heating the preforms, on the run within an oven, to a heating temperature and at a run speed that depend on a programmed heating setpoint, followed by a cyclic phase of forming the preforms within a forming unit including a plurality of moulding stations each equipped with a mould with the shape of a container, the forming phase comprising a pre-blow-moulding step consisting in injecting, into the preforms, a gas with a pressure and a pre-blow-moulding flow rate that depend on a programmed forming setpoint, followed by a blow-moulding step consisting in injecting, into the preforms, a gas with a blow-moulding pressure higher than the pre-blow-moulding pressure, this method furthermore comprising repeating operations consisting in: - measuring, at each point in time, an actual value of the pressure prevailing in the preforms within each mould, at least during the pre-blow-moulding step; - storing the actual pressure values thus measured and the associated measurement points in time in memory; - detecting, among these actual pressure values, a maximum actual value; and - comparing this maximum actual value and the associated measurement point in time with a preset reference maximum value ( P B1) and an associated reference point in time ( t B1), respectively; this method being characterized in that it furthermore comprises the operations consisting in: - checking whether the following necessary conditions are conjointly met over a period longer than or equal to the length of a heating cycle and for all the moulding stations: ∘ the measurement point in time associated with the maximum actual value coincides with the reference point in time ( t B1), ∘ the maximum actual value is lower than the reference maximum value ( P B1); - if these conditions are conjointly met, checking in an event history whether the heating setpoint and/or the forming setpoint were/was not modified in a preset time interval preceding the measurement; - if the result of this check is positive, decreeing the preforms in question to be out of specification; and - taking at least one of the following actions: ∘ generating an alarm, ∘ stopping supplying the oven with preforms, and/or ∘ ejecting the containers obtained from the out-of-specification preforms. - measuring, at each point in time, an actual value of the pressure prevailing in the preforms within each mould, at least during the pre-blow-moulding step; - storing the actual pressure values thus measured and the associated measurement points in time in memory; - detecting, among these actual pressure values, a maximum actual value; and - comparing this maximum actual value and the associated measurement point in time with a preset reference maximum value ( P B1) and an associated reference point in time ( t B1), respectively; this method being characterized in that it furthermore comprises the operations consisting in: - checking whether the following necessary conditions are conjointly met over a period longer than or equal to the length of a heating cycle and for all the moulding stations: ∘ the measurement point in time associated with the maximum actual value coincides with the reference point in time ( t B1), ∘ the maximum actual value is lower than the reference maximum value ( P B1); ∘ the measurement point in time associated with the maximum actual value coincides with the reference point in time ( t B1), ∘ the maximum actual value is lower than the reference maximum value ( P B1); - if these conditions are conjointly met, checking in an event history whether the heating setpoint and/or the forming setpoint were/was not modified in a preset time interval preceding the measurement; - if the result of this check is positive, decreeing the preforms in question to be out of specification; and - taking at least one of the following actions: ∘ generating an alarm, ∘ stopping supplying the oven with preforms, and/or ∘ ejecting the containers obtained from the out-of-specification preforms. ∘ generating an alarm, ∘ stopping supplying the oven with preforms, and/or ∘ ejecting the containers obtained from the out-of-specification preforms.