Lambda sensor

I have a 2008 petrol s85 piaggio porter. the management light indicating faulty front Lambda sensor keeps coming on at around 200 miles usually when the petrol is low. I have replaced it with a genuine piaggio sensor and replaced the after cat with a 50 POund one, still comes on.
Could a false reading hide another problem, it sometimes comes up with coil error codes but always clears itself.
Has anyone had similar problems and found a cure.
Also can anyone suggest a cheaper Lambda sensor replacement that worked for them. If you can let me know the brand and the product number and the supplier they bought it from. All answers appreciated.
Bob