Method for monitoring the state of a device for delivering thick matter (DS), wherein the device has: - a first drive cylinder for receiving hydraulic fluid (HF), - a first drive piston , which is movably arranged in the first drive cylinder , - a first delivery cylinder for receiving and releasing thick matter (DS), a first delivery piston , which is movably arranged in the first delivery cylinder , and a first piston rod , which is fastened to the first drive piston and to the first delivery piston for coupled movement of the first drive piston and the first delivery piston , - a piston seal , which, in the non-defective state, seals off a drive-pump-side volume (V1) in the first drive cylinder with respect to a swing volume (V2) in the first drive cylinder in conjunction with the first drive piston , - a rod seal , which seals off the first drive cylinder with respect to an environment in conjunction with the first piston rod , and - a drive pump , which is designed to generate a drive volume flow (AVF) of hydraulic fluid (HF) for moving the first drive piston in the first drive cylinder , wherein the method has the steps: - determining a speed of the first drive piston , - calculating a difference between the determined speed of the first drive piston and an expected speed of the first drive piston , and - identifying a fault state in the form of a defect in the piston seal and/or in the form of a defect in the rod seal according to the calculated difference between the determined speed of the first drive piston and the expected speed of the first drive piston , characterized by the step - calculating the expected speed according to the generated drive volume flow (AVF). - a first drive cylinder for receiving hydraulic fluid (HF), - a first drive piston , which is movably arranged in the first drive cylinder , - a first delivery cylinder for receiving and releasing thick matter (DS), a first delivery piston , which is movably arranged in the first delivery cylinder , and a first piston rod , which is fastened to the first drive piston and to the first delivery piston for coupled movement of the first drive piston and the first delivery piston , - a piston seal , which, in the non-defective state, seals off a drive-pump-side volume (V1) in the first drive cylinder with respect to a swing volume (V2) in the first drive cylinder in conjunction with the first drive piston , - a rod seal , which seals off the first drive cylinder with respect to an environment in conjunction with the first piston rod , and - a drive pump , which is designed to generate a drive volume flow (AVF) of hydraulic fluid (HF) for moving the first drive piston in the first drive cylinder , - determining a speed of the first drive piston , - calculating a difference between the determined speed of the first drive piston and an expected speed of the first drive piston , and - identifying a fault state in the form of a defect in the piston seal and/or in the form of a defect in the rod seal according to the calculated difference between the determined speed of the first drive piston and the expected speed of the first drive piston , - calculating the expected speed according to the generated drive volume flow (AVF). <EOS>