"are functioning properly" is correct and can be used in written English. You can use it to describe a mechanical or digital device that is working correctly. For example: "The office printer and computer are functioning properly.".
Transponders must be regularly calibrated and checked to make sure they are functioning properly.
Colorado, for example, relies on state assessments of whether existing regulations are functioning properly.
On the contrary, he added, the absence of trials would be a success because it means national institutions are functioning properly and are handling these cases themselves.
Under federal regulations, miners are supposed to visually inspect oxygen packs to see whether the color-coded indicator shows that they are functioning properly.
Not a single oil refinery is functioning properly.
Initial tests concluded that the packs were functioning properly, Mr. Fillpot said.
No such systems were functioning properly at the beginning of the Ebola epidemic.
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Ha Thuy Vy
MA of Applied Linguistic, Maquarie University, Australia