Haha I'm one of those guys who's been buying a bunch of parts...not that I can really afford it. Every time I get frustrated I just keep telling myself "It's either this or a car payment". Anyways p1506 is defined as an IAC overspeed error, meaning the ecm has detected an engine speed greater than desired. Probable causes: engine vacuum leak or failed IAC valve.

To test the IAC:
1. Turn ignition off if its not already
2. Disconnect wiring harness from IAC
3. Use an ohmmeter to measure the resistance between the terminals
4. Resistance should be between 7-13 ohms. If not, clean and try again. If results are the same, you need a new one (about $50).

I have also heard of unbolting the IAC from the throttle body, leaving the wiring harness hooked up, and turning the ignition to the on position without starting the engine. You should see the valve inside the IAC move to the fully open position. This way you can visually confirm that it is working.

Since my last post, I've replaced my IAC and TPS sensors and I still have the same problems. I did find a nice vac leak on one of the lines going from the vac manifold to the intake, but I can still hear another one. I'm about to take the intake apart and check all lines and gaskets for the leak. I also have a complete exhaust to throw on. If its not the IAC i would bet that you have a vac leak somewhere. All this vacuum assisted - sensor dependant - computer controlled CRAP makes me not want to buy a new vehicle. My next one will be something carburated with NO COMPUTER!

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1997 Ford Ranger XLT short bed regular cab
2.3L Lima
5 speed manual