2007 Ford Mustang car

A common cause of those codes on Fords is the EGR valve. Replace it. Don't bother trying to clean it.

To check for vacuum leaks, start with a visual inspection of the hoses for cracks and dry-rotting. You can also spray water over the engine while it's still cool to see if it gets sucked in somewhere. If you have access to a smoke machine from an auto parts store that rents or borrows tools, inject the smoke into a vacuum hose or the intake manifold, then watch if it sneaks out somewhere.

Nope. You get an EGR-related code when the computer detects a problem with the sensor on it. Codes 171 and 174 refer to both sides of the engine running lean, but the computer doesn't know why. We know that a failing EGR valve, (failing mechanically, not electrically), is the most common cause of repeated lean codes.

To say this differently, you'll also get same lean codes if there's a vacuum leak. Again, the computer has no way to detect that mechanical problem. It just knows the exhaust is too lean, but it doesn't know why.

Then you'll have to start with vacuum leaks. To check for vacuum leaks, start with a visual inspection of the hoses for cracks and dry-rotting. You can also spray water over the engine while it's still cool to see if it gets sucked in somewhere. If you have access to a smoke machine from an auto parts store that rents or borrows tools, inject the smoke into a vacuum hose or the intake manifold, then watch if it sneaks out somewhere.

I show an EGR valve for your engine. If there is one, it will be shown on the emissions sticker under the hood.

Also check the fresh air tube between the mass air flow sensor and the throttle body. Any leaks there will let in air that isn't measured, so no fuel will go with it. That will cause a lean condition.