HVAC lines

Hi all, I have a HVAC unit in the garden with a buried line into the house. The repairman said the unit is all out of freon and does not have a leak, so he said it can only be because of an underground leak. The warranty people say they wont cover the line repair because underground lines are not up to code. Is that true?? I could not find anything saying underground HVAC lines are not up to code. I'm in TX.