Radon Still High After Mitigation? Troubleshooting
Your post-mitigation radon test is still at or above 4 pCi/L. Check the test, the fan and the system design, and what the contractor's guarantee should cover.
A well-designed active soil suction system usually works: EPA's cost table credits sub-slab, drain-tile, sump and block-wall suction with 50 to 99 percent reductions, and EPA says most homes can be brought to 2 pCi/L or below. So a post-mitigation result at or above 4 pCi/L means something specific is wrong, and it is usually one of three things: the test, the fan, or the design.
Work through them in that order. Testing problems are the cheapest to rule out, and a design problem is the one your contract's guarantee is meant to cover. EPA's Consumer's Guide notes that many contractors guarantee they will adjust or modify the system to reach a negotiated radon level, so read that clause before paying anyone for more work.
1. Rule out the test
- Timing. EPA says the post-mitigation test should be done within 30 days of installation but no sooner than 24 hours after the system is running with the fan on.
- Length. EPA recommends a 2- to 7-day measurement to check the system's effectiveness, and at least 48 hours for any radon test.
- Closed-house conditions. Windows and doors should be closed for 12 hours before and during the test, except for normal entry and exit (EPA).
- Location. Test in the lowest lived-in level, in the same spot as the original test, so the numbers are comparable.
- Independence. EPA recommends an independent follow-up measurement in addition to any test by the contractor, to avoid a conflict of interest. HomeGuide lists a professional test at $125 to $400 and a do-it-yourself kit at $10 to $30.
2. Rule out the fan and pipe
Look at the system's warning device. A U-tube gauge with even liquid levels means no suction. Check the breaker and outlet, then the exposed pipe for loose joints. If the fan is dead or noisy, see radon fan not working. EPA notes a fan replacement cost about $200 to $350 with parts and labor (2010 dollars).
3. Look at the design
| Finding | What it suggests | Typical fix |
|---|---|---|
| Fan runs, gauge reads, radon barely moved | Poor air movement under the slab; one suction point not reaching the whole footprint | Add a suction point; EPA notes the number of points depends on how easily air moves under the slab |
| High readings near block walls | Soil gas rising inside hollow block cores | Add block-wall suction, which EPA says is often combined with sub-slab suction |
| Part of the house over a crawlspace or slab addition | Mixed foundation not covered by the system | Add a crawlspace membrane or a second slab point; EPA says mixed foundations may need a combination |
| Open sump, floor drains or large cracks | Short-circuiting: the fan pulls basement air instead of soil gas | Seal major openings; EPA treats sealing as a basic part of most approaches |
| Passive pipe only, no fan | Passive suction reduces radon 30 to 70% in EPA's table | Add a fan, about $300 in EPA's 2010 guide (2010 dollars) |
| Private well | Radon released from water use | Test the water; EPA lists aeration and carbon (GAC) treatment |
Reductions, combinations and the fan add-on figure: EPA Consumer's Guide to Radon Reduction (2010).
A contractor can use the same diagnostics EPA describes for new systems: chemical smoke to find air movement at cracks, drains and sumps, and a soil communication test to see how far suction reaches under the slab. Ask for the readings in writing.
What to do, step by step
- Step 1: Retest under proper conditions if the first post-mitigation test broke any of the rules above.
- Step 2: Check the gauge and fan. Fix any power or pipe problem, then wait at least 24 hours and retest.
- Step 3: Call the installer with both test reports and ask them to honor the guarantee. EPA advises that the contract spell out any guarantee to reduce radon below a negotiated level and for how long.
- Step 4: If they will not, get a second credentialed mitigator to diagnose it. EPA suggests checking with your state radon office or consumer protection office about the contractor.
- Step 5: Price any added work against published ranges. A second suction point is usually a modest add-on; a full block-wall system is $1,800 to $3,000 in HomeGuide's 2026 survey. Use the radon mitigation cost calculator and radon mitigation methods compared to check a bid.
After it is fixed
EPA recommends retesting at least every two years to be sure levels stay low, and again after any major remodel. Keep every test report with the system's paperwork; if you sell, EPA says to give buyers information on how the system operates and is maintained.
Questions
Is a result just under 4 pCi/L good enough?
It meets EPA's action level, but EPA notes levels below 4 still pose some risk and most homes can reach 2 pCi/L or below. Ask whether one more suction point is likely to help.
Should I pay for the extra suction point?
Check the contract first. If it guarantees a level and the system missed it, the adjustment should be the installer's cost.
Could the radon be coming from my water?
In a small share of homes, yes. EPA says radon from soil is usually a much larger risk, but homes on private wells should consider testing water as well as air.
If you are starting from a new high result instead, read what to do after a high radon test.
Sources
- EPA, Consumer's Guide to Radon Reduction (2010 edition, archived)
- EPA, A Citizen's Guide to Radon (2016)
- EPA, How much can a radon mitigation system cost?
- HomeGuide, Radon Mitigation Cost (Feb 5, 2026), cited as text
Prices are published ranges from the sources listed on this page; anything we calculated is labeled as an estimate. Get at least two itemized bids from certified or licensed mitigators. Run the calculator to build your own range.