1997 Nissan Altima 1997 Nissan Altima Throttle Problem

I have a 1997 Nissan Altima (manuel). It shows a code of PO120 which is for the throttle position sensor. I replaced the throttle position sensor, but it still has the same code. I don't think that the part was defective because before replacement of the throttle sensor, the vehicle misfired when driving. It's not misfiring anymore, but it is idling too high. When it's in neutral, it idles at 500 rpm. When I place it in gear, it idles at 1000 - 2000 rpm. The gas pedal is also hard to press. It recently had a tune up and I've had the manifold intake checked for leaks and none were found.

I'd appreciate any help on this matter.