Powertrain control memory module

1994 Dodge Conversion Van, B250 5.2 liter, v8. Van was running smooth, no stalling, nothing. Only change I did was go from premium gas to regular due to price. Next morning, wouldn't turn over. Has charge, but it sounds as if no gas is circulating. Took it to dealership. Service write up technician wants to order "PCM" as she says that she can't get reading from her computer and that it will cost about $700.00. Asked her if that was the problem, she said no, that it was just to get the "pmc" installed and then get a reading to see what the problem would be, which would then be additional charges involved to find out what is the problem. My question to you is : I thought that the point of taking the van in for a diagnostic was so that they could put the computer to see what the problem is but now she wants me to pay for a $700.00 part that she also states that she can’t verify that that is the problem, can you explain this to me?