Losing power on hills and having intermittent starting issues during rain.

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Sounds like an ignition misfire. The bad thing about a chronic check engine light is that you never know when additional codes are recorded in the computer. A trip to one of the chain Autoparts stores will get you a free computer diagnostic scan. Then you can see if there are subsequent codes that have come up related to engine performance. Like a misfire code or system lean code... Because of the correlation of "rainy days" and the performance drop under power, I am leaning towards an ignition failure of some sort, but further diagnostics may be needed. BTW, to my knowledge, there is no code that comes up that states to replace the water pump, so you might want to "fact check" that one...