Help, I have a 2005 mercury monterey that had code p0401 on it. I cleaned the EGR valve with carb cleaner and reset code by disconnecting negative battery cable to battery. I connected everything back up, code went away and now 2 weeks later I am getting horrible gas mileage. Why? thanks.
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May not have anything to do with past issue. I suggest starting from scratch and have the codes read. Just because the check engine light isn’t on, it can still have codes stored. Best place to start. Might be as simple as a failed/stuck engine temp sensor and the engine is dumping extra fuel in but checking the codes is your first move.