P1506 Ford -

Question

yes, replaced IAC valve. Could a faulty upstream O2 sensor be the cause of this. Seeing that the upstream sensor controls the amount of fuel entering the car.

Answer 1

P1506 No the 02 sensor can not be the cause of this code condition what you have is a overspeed condition, this can be due to vacuum leaks , which will increase the RPMs.The vacuum leaks can be from a leaking intake manifold gasket to a PCV hose rupture or a open PVC valve, any vacuum leak that will casue a overspeed of the RPM.