I am having trouble with my 2001 Subaru Forester. There seems to be a problem with fuel delivery. From a stop it stumbles. Usually no misfire. I had an engine code for low cat eff and misfire on cyl 3. did some checking with a snapon scantool and found the front o2 sensor was reading just out of parameters so I replaced it. Problem still continued. Did the test for Mass airflow and MAP sensors check injector on cylinder 3 and found no problems. when looking at cyl 3 found the plug to be white. Switched the cyl 3 and 1 injector and add injector cleaner. Replaced fuel filter and still have the same problem. Any help on this would be helpful.

You swapped injector with cyl 1 but the problem stayed with cyl 3 correct?

Did you check the rest of the plugs as well? Do that if you haven't.

White on the plugs generally means a lean condition, or ash deposits from burning small amounts of coolant or oil or some other form or foreign material in the combustion chamber.

When was the last time it got new plugs and wires? Were/are the wires Subaru wires?

x2 on plugs and wires. Go with the simple stuff first rather than injectors and everything else. They are simple motors, with simple problems 9x out of 10. NGK BKR6E11, don't let anyone tell you otherwise. P0420 isn't going to cause a running condition, so don't worry about that. Cyl 3 misfire is the easiest clue you can get. Check your connections on that plug wire. Make sure that plug well isn't loaded with oil from bad cam cover gaskets. And try changing the plugs. You can also switch that cylinders plug wire with a known good one from another cyl and see if the misfire follows that wire. Easy stuff first

as an added thought - try pulling each wire, one at a time, with the car running to check for bad wires - i had one that had started arcing out the side of the plug boot! it was VERY obvious when i pulled that wire with the car running...

Plugs and wires were changed along with the fuel filter. The hesitation is not always there. Generally there is not a misfire just a hesitation. Definitely feels like a fuel delivery problem. The problem is not as bad after swapping injectors. I also ran a bottle of injector cleaner through with the last tank. There are no oil leaks on the car at all. A small power steering leak I just noticed while working on this problem but that is on the other side of the engine.

Have you checked to see if the throttle position sensor has a dead spot at the low end of it?

Ever used Seafoam?

Plugs and wires were changed

aftermarket wires can cause cylinder misfires with these engines. EJ engines are notorious for it, i've seen new wires right out of the box cause misfires.