An elevator installation, comprising: an elevator group controller; at least one input device for issuing an elevator service request; a modernization overlay (4A, 4B, 4C) communicatively connected to the group controller and/or to the at least one input device; at least one elevator (5A, 5B, 5C) communicatively connected to the overlay (4A, 4B, 4C); wherein the overlay (4A, 4B, 4C) is configured for transferring elevator control signals between the at least one elevator (5A, 5B, 5C) and at least one of the group controller and the at least one input device; characterised by: wherein the elevator installation comprises a diagnostics function configured to determine an operational anomaly of the at least one elevator (5A, 5B, 5C) based on: at least one elevator control signal to be sent from the group controller and/or from the at least one input device to the overlay (4A, 4B, 4C) of the at least one elevator (5A, 5B, 5C), and at least one response criteria associated with the response from the at least one elevator (5A, 5B, 5C) to the at least one control signal, wherein the at least one response criteria comprises receiving a response within a given time period from the at least one control signal; and wherein the diagnostics function is configured to determine an operational anomaly of the at least one elevator (5A, 5B, 5C) in case a response is not received by the overlay (4A, 4B, 4C) within a given time period from the at least one control signal. an elevator group controller; at least one input device for issuing an elevator service request; a modernization overlay (4A, 4B, 4C) communicatively connected to the group controller and/or to the at least one input device; at least one elevator (5A, 5B, 5C) communicatively connected to the overlay (4A, 4B, 4C); wherein the overlay (4A, 4B, 4C) is configured for transferring elevator control signals between the at least one elevator (5A, 5B, 5C) and at least one of the group controller and the at least one input device; characterised by: wherein the elevator installation comprises a diagnostics function configured to determine an operational anomaly of the at least one elevator (5A, 5B, 5C) based on: at least one elevator control signal to be sent from the group controller and/or from the at least one input device to the overlay (4A, 4B, 4C) of the at least one elevator (5A, 5B, 5C), and at least one response criteria associated with the response from the at least one elevator (5A, 5B, 5C) to the at least one control signal, wherein the at least one response criteria comprises receiving a response within a given time period from the at least one control signal; and wherein the diagnostics function is configured to determine an operational anomaly of the at least one elevator (5A, 5B, 5C) in case a response is not received by the overlay (4A, 4B, 4C) within a given time period from the at least one control signal. wherein the elevator installation comprises a diagnostics function configured to determine an operational anomaly of the at least one elevator (5A, 5B, 5C) based on: at least one elevator control signal to be sent from the group controller and/or from the at least one input device to the overlay (4A, 4B, 4C) of the at least one elevator (5A, 5B, 5C), and at least one response criteria associated with the response from the at least one elevator (5A, 5B, 5C) to the at least one control signal, at least one elevator control signal to be sent from the group controller and/or from the at least one input device to the overlay (4A, 4B, 4C) of the at least one elevator (5A, 5B, 5C), and at least one response criteria associated with the response from the at least one elevator (5A, 5B, 5C) to the at least one control signal, wherein the at least one response criteria comprises receiving a response within a given time period from the at least one control signal; and wherein the diagnostics function is configured to determine an operational anomaly of the at least one elevator (5A, 5B, 5C) in case a response is not received by the overlay (4A, 4B, 4C) within a given time period from the at least one control signal.