2003 Chevy S-10 2003 ZR2 No Power

This vehicle sat outside for over a year without being driven. We changed the oil and all the filters, new plugs and wires right after we bought it. It doesn’t have any power on the hills. When I accelerate my speed it seems dead and the check engine light will come on sometimes. When I shut off the truck my check engine light stays off. I wonder if the code is still able to be retrieved by the dealership if the check engine light is out? The truck is using more gas as time goes on. Also it was running a little hot so my husband changed the thermostat to let it run cooler. We bought a new fuel pump, and clutch fan hoping this would solve the problem. We made these changes one step at a time over the last year we have owned this vehicle, but have seen little improvement. Our next choice is a catalytic converter which is very expensive. My last hope is asking someone with experience with these matters to help, so we hope you can. Thank you.