I have a shower & commode, plumber forgot to put a vent in line. With water in trap in shower, the commode will not flush. If we blow the water out of shower trap, commode works fine. Do you have any other suggesting other than digging up the 2 inch line? What size vent pipe do we need to put between the shower & commode in the 2 inch line?

Plumbers usually don't forget to install vents. They know their importance...

Even w/o vent, toilet still should flush, siphoning shower trap in the process...

Toilet needs 2" vent, shower needs 1 1/2" vent minimum. If you post a sketch of your bathroom, including the rough-in, we will tell you how to proceed with correcting your venting problem...