Should I pay this guy?

So a few months ago I got a new radon mitigation professionally installed in my basement and paid him $1400. About a month after that, my levels were spiking, and had a 12 pci average. So the guy came back and sealed both of my sub pumps. A few weeks go by and the levels are still very high in my bedroom and my daughter's bedroom. I messaged him and he said he will probably have to put in a whole new system in the attached garage. Now he is trying to charge me an extra 800 for this garage system. The original agreement clearly states that he gurenteed a below 4pci for 5 years and that if the post job inspection was above 4 I would not have to pay for any upgrades to get these radon levels down. I feel that I shouldn't have to pay. Thoughts?