I have a 2001 4.0 E series. I am getting a p1506 error I changed the spark plugs it didn't help. What should I do next? It doesn't idle that bad it is just a sporadic miss. I can hear the motor rev when listening to AM radio. Thanks

 

On the front of the upper intake where the air tube to air filter connects is the IAC(idle air control) Valve.

Fuel injected engines can't use an "idle screw" because there are no Jets, like a carb has, so there is no way to balance the air/fuel mix with a screw adjustment.
Fuel injected engines all use IAC Valves because of this.
The computer controls the fuel injectors and the IAC Valve so air/fuel can be balanced.

p1506
- Faulty Idle Air Control (IAC) valve
- Idle Air Control (IAC) valve harness is open or shorted
- Idle Air Control (IAC) valve circuit poor electrical connection
- Intake leaks
- Stuck PCV Valve

But before you start replacing parts you need to test some things.
First find the IAC Valve, it will have a 3-wire connector
After engine is warmed up unplug that 3-wire connector, IAC Valve should close and idle should drop down to 500rpms or engine may even stall, either is good it means no vacuum leak.
If idle stays high then you probably have a vacuum leak.
The 4.0l(and 3.0l) are notorious for leaking PCV Valve hoses, at the elbows.
PCV Valve hoses are the larger vacuum hoses, follow each one and run your fingers under and over each hose