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Ron, ASE Certified Technician
Category: Ford
Satisfied Customers: 35603
Experience:  23 years with Ford specializing in drivability and electrical and AC. Ford certs and ASE Certs
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My 2004 ford Taurus has check engine light that says both

Customer Question

Hi, my 2004 ford Taurus has check engine light that says both banks running lean. The mass air flow sensor was replaced, but this still didn't fix the problem. Usually at least once a day, the engine revs while driving and I have to drive while using the brake rather than the gas pedal. Are these problems related?
JA: Does the light stay on solid or does it flicker? Any noticeable performance problems lately?
Customer: The light stays on. No other problems other than the engine revving, and will occasionally stall while driving very slowly through playgroung zones.
JA: Are you hoping to fix this yourself? What have you tried so far?
Customer: The school mechanics teacher where I work has tried replacing the mass air flow sensor. He is thinking of replacing the lower manifold intake gasket. I was just looking for a second opinion.
JA: Anything else you think the mechanic should know?
Customer: No
JA: OK. Got it. I'm sending you to a secure page on JustAnswer so you can place the $5 fully-refundable deposit now. While you're filling out that form, I'll tell the Ford Mechanic about your situation and then connect you two.
Submitted: 1 year ago.
Category: Ford
Expert:  Ron replied 1 year ago.

Hello and thank you for your question,

CAn you give me the actual fault codes?

Which engine do you have please (8th letter of your vin)?

Customer: replied 1 year ago.
8th letter is U so I guess a Vulcan engine and no DOHC? I don't know the code numbers
Expert:  Ron replied 1 year ago.

Yes your is a Vulcan. Where the code P0171 and P0174?

Customer: replied 1 year ago.
Expert:  Ron replied 1 year ago.

One very common cause for these codes to set providing you have good fuel pressure can be caused by a rotted out and leaking PCV supply line and or the vent line. I would start with inspecting both for any signs of a leaking line. See there locations in the links below, copy and paste the links to your browser to open and view them>