9/10 it will be the front lambda sensor. Once the EML light comes on then it's pretty well had it. You can't clean it either so a new one is needed.
If you have a engine fault code reader, then you may be able to see the O2 lambda reading live data and it will show you a value to check against. If its too high, then its a replaceable item.
Also, to remove the lambda sensor, you're better off spraying some plus gas into the bolt joint to the exhaust and letting it soak in for 30 mins or so. This will make it much easier to loosen off the nut.