Device for monitoring a trailer connector socket of a vehicle at which the electrical system of a vehicle trailer can be connected, comprising a circuit - with a first connection (VBB) for connecting to a positive potential of a vehicle electrical system voltage, - with two connections (BrkL, PosLL, PosLR, TurnSL, TurnSR, RevL, FogL) for connecting to contacts of the trailer connector socket, - with a current and/or voltage sensor (SI), - with first controllable switches (S11-S17) each of which are in series to one of the two connections (BrkL, PosLL, PosLR, TurnSL, TurnSR, RevL, FogL), - where each of the two connections (BrkL, PosLL, PosLR, TurnSL, TurnSR, RevL, FogL) is connected via said first switch (S11-S17) with a first node (K), the first switch (S11-S17) connected in series to the second connection (BrkL, PosLL, PosLR, TurnSL, TurnSR, RevL, FogL) being in a closed state and - where the first node (K) is connected to the first connection (VBB) via the current and/or voltage sensor (SI), characterized in that - the first node (K) can be connected via a third controllable switch (S3) and a reference resistor (Rrf) wired in series to a third switch (S3) to a third connection for connecting to a ground potential pf a vehicle electrical system and - when the first switches (S11-S17) are open and the third switch (S3) is closed a current path is created between the first connection (VBB) and the third connection via the first node and the reference resistor (Rrf) through which a current for calibrating the current and/or voltage sensor (SI) and for testing the proper functioning of the circuit can flow which can be measured by the current and/or voltage sensor (SI). - with a first connection (VBB) for connecting to a positive potential of a vehicle electrical system voltage, - with two connections (BrkL, PosLL, PosLR, TurnSL, TurnSR, RevL, FogL) for connecting to contacts of the trailer connector socket, - with a current and/or voltage sensor (SI), - with first controllable switches (S11-S17) each of which are in series to one of the two connections (BrkL, PosLL, PosLR, TurnSL, TurnSR, RevL, FogL), - where each of the two connections (BrkL, PosLL, PosLR, TurnSL, TurnSR, RevL, FogL) is connected via said first switch (S11-S17) with a first node (K), the first switch (S11-S17) connected in series to the second connection (BrkL, PosLL, PosLR, TurnSL, TurnSR, RevL, FogL) being in a closed state and - where the first node (K) is connected to the first connection (VBB) via the current and/or voltage sensor (SI), - the first node (K) can be connected via a third controllable switch (S3) and a reference resistor (Rrf) wired in series to a third switch (S3) to a third connection for connecting to a ground potential pf a vehicle electrical system and - when the first switches (S11-S17) are open and the third switch (S3) is closed a current path is created between the first connection (VBB) and the third connection via the first node and the reference resistor (Rrf) through which a current for calibrating the current and/or voltage sensor (SI) and for testing the proper functioning of the circuit can flow which can be measured by the current and/or voltage sensor (SI). <EOS>