Had same drama with the wifes. Firstly check the connection under rear nearside wheel arch (its blue and black) clean both male and female connectors you can use wd40 i find acf 50 to be the best to protect from moisture. Then start up test if no joy will be a faulty sensor.
To test them get someone in the drivers seat car running and in reverse. Then put your ear to each sensor there's 4. You should hear a faint clicking from each one like a bat. No clicks sensor dead. Peugeot charge £92 +vat for one sensor. They are availble on ebay for £12.99 delivered. Good luck.