I have a high engine rev I know the Iac valve is the obvious answer but this is the only code coming up on my truck, but the idle control on the throttle body had been altered I know this is factory set, but I know there is a way to get it back close to what it is suppose to be. I was hoping somebody might be able to help me out with the details would be most appreciative thanks in advance

 

P1506 Idle Air Control Overspeed Error

It means engine RPMs are higher than expected and computer can't lower RPMs using IAC Valve

Warm up engine, let it idle
Unplug 2 wire connector on IAC Valve
If IAC Valve was was opened all the way then it should close and RPMs will drop to 600 or so, barely running
If RPMs stay high then either you have a vacuum leak, most common, or IAC Valve is stuck open, not working

Not sure what you mean by "throttle body has been altered"
There may be an "anti-diesel" screw on the throttle linkage, but not sure its there in 2001, it looks like an idle screw, and does the same think, it sets the throttle plates Closed limit
You set this with engine warmed up and IAC Valve unplugged, turn it counter clockwise until engine is barely running, 500-600rpm, its set
Plug IAC valve back in and idle should go up to 750 or so


TPS, throttle position sensor, when this fails it can hold throttle open farther than it should, extreme long shot

High idle 99% of the time is a vacuum leak