Crank no start

This issue has been going on for a couple months now, and it sat at a shop, and would give them nothing to test, the only codes that come up are for the O2 sensor. It mostly started to happen in the cold, the car would crank and crank, but would not start, it starts with some starting fluid than runs very well, no stutter, no loss in acceleration, or misfires of any sort. The shop said they had tested everything they could think of, including inquiring with a Hyundai mechanic. Fuel pressure was said to be okay, spark they said was good, good compression as well. The fuel pump primes as normal when key is turned on. They followed the harness for corrosion and bad grounds, and even checked the bottom of the fuse/relay box in the engine compartment, which all proved to be fine. The Hyundai mechanic suggested something with the fuel pump, but my shop did not want to aimlessly replace parts for obvious reasons. So now I am here, hoping someone has experienced this with some luck, or someone knows what this could possibly be.