CRJ 700 flight crew reported being dispatched with MEL for a faulty left wing anti ice sensor. While in cruise they had EICAS messages relating to the wing anti ice system so they decided to return back to the departure airport.

Narrative

We were dispatched with MEL 30-XX-XX-XX for a faulty left wing anti ice sensor being deferred. We took off and flew with the wing anti ice cross bleed set to from right" as called for by the MEL. During climb and initial cruise we did not encounter icing and did not turn on the wing anti-ice. At some point in cruise with the wing anti-ice OFF the wing anti-ice fail red warning EICAS message came on with the associated chime. The Captain called for the QRC/QRH. The procedure essentially said to turn the wing cross bleed valve to the unaffected side. It was already there in compliance with the MEL. The red warning EICAS message then went away after a couple of minutes. As we discussed what it could mean and what to do; the red wing anti ice fail warning EICAS message came back on and then went off after a minute or 2 three more times. All with the wing anti ice off. To see what the system would do; we turned the wing anti ice ON. In a minute or two the amber wing anti ice caution EICAS message came on and the anti-ice display page showed the left wing was amber indicating a failure. Given the red warnings with the system off and the amber caution with the system on we were not confident that we could determine if the wing anti ice system was functioning. Given that ZZZZ was at 1deg; ceilings were at minimums; and there was a SIGMET for possible severe icing just North of our route into ZZZZ; we did not feel that continuing to the destination was safe. In consultation with Dispatch we decided to divert back to ZZZ where we knew there was little to no risk of icing. We proceeded to ZZZ safely without further incident."