Fouling in heat exchangers is the buildup of deposits on the solid surfaces. These deposits reduce the eco-efficiency of the processing equipment and increase the risk of subsequent surface contamination with the formation of biofilms. In the agro-food and water supplier sectors, which are our main concern, fouling on the hot walls of processing heat exchangers is a common occurrence and requires frequent cleaning cycles to ensure hygiene requirements are met. This results in a considerable ecological footprint. Sensors and diagnostic tools for monitoring fouling are thus of utmost importance to ensure the rational validation of the cleaning end-point and to decrease the environmental impact of the cleaning cycles. In this paper, a non-destructive ultrasonic monitoring technique using coda waves and the associated signal processing was tested to monitor the evolution over time of a deposit layer on a solid wall during cleaning. To ascertain the feasibility of the method, a piece of wax of controlled thickness was deposited to simulate the initial fouling state and a cleaning cycle was launched. The decorrelation coefficient was used as an indicator to monitor fouling. This article presents the principle of this unprecedented technique for measuring the degree of fouling. The results of the experiments show that this non-destructive monitoring technology is sensitive to changes in fouling and that the decorrelation coefficient curves are in agreement with the cleaning kinetics captured using a video camera, thus ascertaining the pertinence of the diagnostic tool proposed.