A self-diagnosing FMCW radar level gauge for measuring a distance to a surface of a product contained in a tank, said radar level gauge comprising: a transceiver arranged to generate and transmit an electromagnetic transmit signal in the form of a frequency sweep; a signal propagating device; a signal propagation path connecting said signal propagating device to said transceiver; wherein said signal propagation path and said signal propagating device are configured to guide said electromagnetic transmit signal towards said surface, and return an echo signal including reflections from said surface and an impedance transition in the signal propagation path; a mixer connected to said transceiver and configured to mix said echo signal with a portion of said electromagnetic transmit signal to provide an intermediate frequency signal; a filter arrangement connected to said mixer and configured to filter said intermediate frequency signal in order to provide a filtered intermediate frequency signal; and processing circuitry connected to said filter arrangement and configured to process said filtered intermediate frequency signal; wherein said transceiver is arranged to output said frequency sweep as a diagnostic sweep, said diagnostic sweep is configured such that a reference echo is detectable in said filtered intermediate frequency signal, said reference echo being indicative of a distance to said impedance transition; said transceiver is arranged to output said frequency sweep as a measurement sweep, said measurement sweep is configured such that said reference echo is suppressed in said filtered intermediate frequency signal, and that a surface echo is detectable in said filtered intermediate frequency signal, said surface echo being indicative of a distance to said surface; and said processing circuitry is configured to self-diagnose said radar level gauge based on the reference echo, and to determine the distance to said surface based on said surface echo. a transceiver arranged to generate and transmit an electromagnetic transmit signal in the form of a frequency sweep; a signal propagating device; a signal propagation path connecting said signal propagating device to said transceiver; wherein said signal propagation path and said signal propagating device are configured to guide said electromagnetic transmit signal towards said surface, and return an echo signal including reflections from said surface and an impedance transition in the signal propagation path; a mixer connected to said transceiver and configured to mix said echo signal with a portion of said electromagnetic transmit signal to provide an intermediate frequency signal; a filter arrangement connected to said mixer and configured to filter said intermediate frequency signal in order to provide a filtered intermediate frequency signal; and processing circuitry connected to said filter arrangement and configured to process said filtered intermediate frequency signal; wherein said transceiver is arranged to output said frequency sweep as a diagnostic sweep, said diagnostic sweep is configured such that a reference echo is detectable in said filtered intermediate frequency signal, said reference echo being indicative of a distance to said impedance transition; said transceiver is arranged to output said frequency sweep as a measurement sweep, said measurement sweep is configured such that said reference echo is suppressed in said filtered intermediate frequency signal, and that a surface echo is detectable in said filtered intermediate frequency signal, said surface echo being indicative of a distance to said surface; and said processing circuitry is configured to self-diagnose said radar level gauge based on the reference echo, and to determine the distance to said surface based on said surface echo.