Gas exhaust goes to the probe, water is just a condensation drain. Take the filter assy apart and see how bad it is, leave it out if unsure and put it back together and see if you have suction into the exhaust hose, wont be a lot of it. If so it might work and the guy was wrong. A good way of checking the meter is use a small plumbers torch with propane and hold it to the cal inlet and put the switch to cal. of course unlit right! The meter should register rich or at least move around. Forgot what the gas they used was might have been propane/nitrogen.