I bought a 4 cylinder Sonata with 120,000 miles. The PO171 code was on when I got the car. New oxygen sensors were put in before I got the car. I changed the fuel filter, ran every kind of fuel injector cleaner through, another code came up - insufficient engine coolant temp. The car has problems climbing hills and seems to have problems shifting on hills. Also, when cold the car stumbles and has a total lack of any power, unless you let up completely on the gas pedal and you have to do this several times. A mechanic suggested I put in a new fuel pump because the old one was weak. I changed the fuel pump (was a cheap one I bought online - may not be a good one). He also suggested changing the t-stat (which I did) and he said the timing was off a bit. I changed both timing belts but didn't install correctly and ran the car with the timing off. I had to correct the timing. I am afraid I may have damaged valves, however that was 2,000 miles ago. Right before I changed the timing belts, a PO300 code came up and camp up again after the timing belts were changed. A mechanic suggested new plugs and wires (which I did). Every time I made these little repairs the car seemed to do a little better, but codes still keep popping up and the car still has problems on hills, accelerating, and shifting. Just recently I pulled up four codes: PO171, PO335, PO300 & PO725. I will also add that I changed the trans. Fluid when I first got the car and used lucas trans fix. I know there is a small trans. Leak at the radiator. I have not checked the EGR. Can you please give me some suggestions?