In the thermal update, as recently as March 26, occasionally air pumps would stop working and display the status "Not pumping." This was with power on and connected, full pressure at the pump end, and almost vacuum next to the exhaust vent (will all gas pipes properly connected as well). Deleting the pipes and adding another gas pump did not fix the problem (the new pump stopped working immediately as well). Once I restarted the game, the pumps immediately started running again. (Now to deal with the several hundred kilos of contaminated water that my dupes made while stressing over the lack of oxygen ).
Connect a gas pump in a high oxygen pressure area (such as above an electrolyzer), and run a gas pipe and a vent outside the room to a low pressure area (possibly a vacuum). Allow it to run for a while (~20 minutes). The bug is random, so it may or may not stop pumping.