Engine light

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The check engine light is on. Mileage 73,000. My local mechanic has replaced the hose assembly and gasket. I am told PSI is 22, and it should be at least 40. Now, they want to replace the fuel filter and possibly the fuel pump. Am I paying my mechanic to continually try to diagnose the real issue. He wants $ 700 to replace the fuel pump and I dont want to invest that kind of money now. Thank you ERROR CODE IS P00171, BANK ONE IS TOO LEAN