My parking sensors stopped working and parallel parking is torture

I rely so much on my parking sensors especially when squeezing into tight spots in the city. They suddenly stopped working and now I'm back to guessing and praying every time I parallel park. It's stressful and I know it's only a matter of time before I bump something. I took it to a shop and they quoted me a fortune for a whole new system which seems excessive. While researching alternatives I found an article about improving car performance in urban areas that actually had a section on sensor maintenance and troubleshooting. The guide explained that sometimes it's just dirty sensors or a loose connection not a full system failure. The information saved me from spending money I don't have. Has anyone else dealt with sensor issues and found a simple fix or am I eventually going to have to pay for replacement?