I have a 2001 F-150 (5.4L). When I start the truck it starts in a high idle then climbs to 2500RPM in about 2 minutes. If I shift gear to either reverse or drive the RPM drop down to about 1100RPM.
So in park and neutral the truck idles at 2500 or higher.
If I let the truck run(normally in either drive or reverse with foot on brake) for more than 5 minutes when I turn the truck off it will not start again for at least 2 hours or so.
I used a simple code reader and it gave the P0155 code and that's the O2 sensor on the bank 2.
I have cleaned the throttle body and replaced the TPS
I have cleaned then replaced the IAC motor
I have cleaned the MAF sensor. I also checked the MAF and it seems to working fine.
Is the next step to check for vacuum leaks or something else?

 

No, your next step is to check the wiring on bank 2, front sensor and see if it is damaged or has a fray, etc. If not, then the O2 sensor is most likely bad or going bad. These are HEGO sensors with 4 wires, so they aren't the cheapo's of the late 80's-early 90's. You want to make sure that this is your problem. You can try a used one (if you have one) or you can clear the codes and switch the right and left sensors (frt ones) and see if the codes switch to P0135. If not and you still have the same code, then your wiring to it is most likely your culprit. (This is just the poor mans way of checking, but with the costs of HEGO's, it pays to check all the options before spending the money for something you may not need). Good luck.

 

I checked the oxygen sensor and it was blown. So I changed the sensor out with a new sensor. Still extremely high idle in park and neutral around 2200rpm. Drive and reverse idle around 1100rpm.
I totally forgot to check the harness to the sensor. So I guess both the sensor and the harness was bad.
Could I have a fuel pressure issue? Because the vehicle wont restart after it has ran for more than 5 minutes. If I press the gas pedal it will restart

 

BTW Thanks for the help

 

It sounds like the IAC valve but you say that you have replaced this valve. If you can check the voltage to this valve, it should read from 10.5 to 12.5 volts. It would not hurt to check the valve itself, the resistance across the terminals of the valve should be between 6 and 13 ohms. When you installed the new valve, was there a lot of carbon in the valve? It might be a good idea to clean out the ports around the valve and then reinstall your new valve.

A simple inspection of the wiring harness to your Oxy sensor should be sufficient. Usually this is obvious damage if it is the cause of your problem and the most common problem is a bad oxy sensor. If the 155 code does not come back on you are most likely good to go. First fix the idle because this might be causing the Oxy to kick on with the code.

It is always good to check for vacuum leaks and fix. change the PCV valve as this is low cost and can often cause idle problem and leaks in the hose to the PCV. A new PCV only cost about $5. Get one with the rubber grommet that fits into the valve cover for a little more and change that hose to the pCV if it looks bad. Change your fuel filter also (less than $10) to avoid fuel problems. Clogged filter can make you fuel pump work harder and cause early failure.

 

I checked the voltage to the iac motor and it only reads 2 volts and not the 10.5-12 volts required.
So I'm looking for a PCM diagram for my truck so I can test the output from the PCM. I don't have a fuse diagram either but I did check all the fuses all were good