1997 Ford E350 DTC: P1151

My 1997 ford e350 6.8L Triton V10 has just turned on the check engine light and i scanned it and got one dtc: P1151. I know what the code means but what could be causing it? It has 90,000 miles on it and the only work it had done it was to plug a hole in the fuel rail. Someone broke off the fuel pressure test valve and i couldnt find a replacement so the mechanic retapped and plugged the hole.

Please help, Thanks

What is DTC P1151? I would like to help you understand what is causing it.