My radon manometer has a red reading, what should I do?
To check if the fan is working, you can put a plastic bag over the end of the exhaust pipe and see if it inflates.

Gauge oil is just the orange fluid that goes in the manometer (pressure meter) tube that indicates the pressure difference being driven by the fan. If it is missing or low, then it could give the negative reading. It should go to zero when the fan if off (as long as the wind isn't blowing too hard).

Even if the bag inflates, it may not be creating a strong enough pressure difference, so you should make sure there is the proper amount of gauge oil in the manometer.