I have a 98 F150 4.6L with just under 30K miles...yep. This last week I get a CEL and read code P0401 Insufficient air flow. I spend the weekend chasing it down and now I get PO123 TPS circuit high input. According to the Haynes manual you can only chech to see if the TPS is good or not by voltage readout. The voltage on the stock TPS was out of spec so I replaced it. After clearing the code the CEL comes back on with the same code and the transmission will not upshift until 5K rpm under normal driving. The Haynes manual is a bit uninformative. Any ideas?

 

The high circuit input reading means that the TPS is not grounded. You need to check the wiring harness.