If its the 45RFE or 545RFE trans then it can happen as the main filter in the pan (it has 2 filters) pushes up into the valve body and has a seal that pushes in the valve body first then the filter neck pushes into the seal which is a metal outer coated housing so the seal ring stays pressed in the valve body. But may times I have seen either a tech did not install the seal right in a trans service or the factory did not get it in right and the filter neck does not seat right and just sucks in air since the filter neck is above the fluid level. The filter can fall out or its just not sealed because the seal is not in right and it does not suck any fluid into the pump because its just pulling in air from the loss of the sealing at the filter neck to the valve body and it will of course set that code since the pressure switch will see no trans fluid pressure. Ron