Should I test for radon when the home already has a mitigation system?
Yes. A mitigation system is equipment, not a guarantee, and equipment fails. The only way to know a system is actually working is to test.
Systems fail quietly
A radon mitigation system is a fan and vent pipe that pull radon from beneath the foundation. It works well when it runs, but a few things go wrong in real houses:
The fan dies. Fans run around the clock and typically last 5 to 10 years. When one quits, it makes no noise and leaves no puddle.
Someone unplugs it. Painters, remodelers, and crawl-space work all lead to unplugged fans that never get plugged back in.
Piping cracks or comes apart, usually in the attic or at a joint, and the suction path is gone.
The home changes. Additions, finished basements, and foundation work can open new paths for radon that the original system was never designed to handle.
With the fan off, the system is just a pipe. It may vent a little passively, but you are no longer getting the protection it was installed for.
What the experts recommend
The EPA and Colorado's health department recommend retesting every two years, even in homes with a mitigation system, and again after any renovation. That recommendation exists precisely because systems drift and fail without anyone noticing.
Buying a home that already has a system
A listing that says "radon mitigation system installed" tells you a system exists. It tells you nothing about whether it is running today. Before you rely on it, test. The test is the same 48-hour monitor, run with the system operating, and it is smart to check the fan and the little gauge on the pipe first. If the gauge shows no suction, the fan is not moving air.
The good news
Testing a mitigated home is quick and inexpensive, and most of the time the result comes back low and you have real proof the system works, not just a pipe in the corner. When it does come back high, the fix is usually simple: a new fan, a reconnected pipe, or an adjustment by a mitigator.
If you are in the Denver metro
I am Bard Nielsen, owner of Golden Peak Inspection in Golden, CO, and I am NRSB certified for radon measurement. I run 48-hour radon tests across the Denver metro, including homes with existing mitigation systems. Call or text 303-803-5412.