Ford,Recently replaced the TPS but still get code P0122, what causes this low voltage?

The code a I get (P0122) is for a low voltage at the TPS wiring plug. The TPS is new and the vehicle has just had new CATs fitted. It runs ok (slightly erratic idle), but the engine warning light comes on so I cannot sell the vehicle: HELP please. I have checked every forum known to man and they don't seem to know what causes this.

My car has 157000 miles.
My car has an automatic transmission.