I had problem with car cutting out at idle.

I had problem with car cutting out at idle thought too be clogged fuel filter which was replaced (confused as replaced separately yet seen a lot of info saying filter is part of fuel pump). Anwyay spark plugs also changed and throttle cleaned. Cutting out problem fixed but now has hesitation and popping noise from exhaust. Check light been on twice same both times code for O2 sensor stuck lean.
Garage involved sayes minor problem and nothing to worry about and cleared codes. Not happy as I'm scared running it like this as it will cause other problems. Help!