S7O LOST POWER/Check engine light

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Help from anyone please, my 98 s70 went into the shop, for check engine light, the mech said it was a solenoid that i needed to replace when i picked it up,i started it up and make a weird noise but stopped after it warmed up the check engine light was gone, the car had great power,the next day the light came back on and i am back to lost power and gas mileage stinks, can someone give me an answer, i have to bring car back in monday

Barry

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