Ok this is my ongoing problem:
1999 fordf150 83k miles
Replaced both cat converter
Computer replaced
Replaced shifting solenoids
codes p0750 & p1504 come up still
Difficult to start and running rough.
I've towed my tk up from ca where they couldn't fix it. Now here in wa my mech can't figure out whats wrong with it either. Spent $900 so far and engine light is still on.
Any suggestions??

 

Probably the IAC.
Grab your DMM.
With KOEO, how many volts do you have at the IAC connector?
How many ohms do you have across the IAC?

And you have an open in the "A" Shift Solenoid circuit wiring. (In other words, the PCM didn't detect a voltage drop through the SSA circuit. Were there any pins pushed back when you installed the new PCM?)

 

What the specs for testing the iac?

 

The 1504 code is an issue with the IAC circuit.
Open, short etc. The IAC resistance is speced at 6 to 13 ohms, a low value.
Test it with both polarities from the meter.
You may see a reading of one value in one direction and a bit different in reverse. it will be normal. The IAC may have a diode accross the winding for spike protection making it read different in each direction.
.
The 750 code is is trans solenoid 'A' error.
Sounds like the shop that did the work was not very well versed.
I would wonder why the change in computer and a cat assembly unless there was supporting evidence for the expense, beyond guessing.
Good luck.