We are using Agilent 7890B GC/FID with Headspace sampler. There is a problem in FID unit of the system. In FID parameters, Temp is 220 C, H2 fuel flow is 30 mL/min, Makeup flow (N2) is 25 mL/min, He flow 1 mL/min. These are setpoints and actuals are also the same. The problem is in air flow. We set the air flow as 400 mL/min but it is still staying at -4 mL/min. In other words, no air is coming to detector (or to the air sensor). We are using Nitroair generator (claind) for nitrogen and air feeding. It is working properly, producing air of 80 psi. There is no leak in air pipe between instrument and source. What can be the reason?