Onboard device and method for checking DEF quality

An onboard device 10 for checking the quality of DEF (Diesel Exhaust Fluid) 20 in a work machine comprises a flow channel 12, which fluidly connects a DEF inlet 14 with a DEF tank 100, and a DQS (DEF Quality Sensor) 16 configured to be brought into contact with DEF passing through the flow channel. The flow channel may have a U-shaped section (18, fig 2) where the DQS is submerged in a retained amount 20' of DEF. The flow channel may have a bypass 120 with a bypass reservoir 123 and a bypass valve 124, and the DQS may be downstream of the bypass valve or in the bypass reservoir. A method of checking the quality of DEF comprising the steps of filling (S10) DEF into the flow channel and checking (S20), by means of the DQS, the quality of the DEF in the flow channel.