Whether you're talking about the Ford Taurus or any other car produced since 1996, code P0174 always indicates the same sort of problem. This code is a generic "emissions" listing, used to help federal emissions officers determine whether or not your car is spewing cyanide into the groundwater. While this very common code always means "lean condition, bank two," the causes for a fuel-poor condition can be extremely varied.
Take your car to a certified Ford mechanic and have all the active and stored codes checked. A generic Onboard Diagnostics, Series II scanner only picks up certain codes relevant to the car's emissions output; it's not really designed to provide precise diagnosis. More sophisticated scanners can read manufacturer-specific and stored codes that a generic scanner may not, and that kind of information can save you a great deal of time and money in tracking down problems. But, you'll need to clear the codes after checking them, regardless of how you do it.2
Locate the primary oxygen sensor and unplug it. If you're facing the "front" of the engine, the side with the belt and alternator on it, bank two is on the right -- driver -- side -- of the engine. Check that the sensor connector harness is tight; a "lean" signal from the O2 sensor is essentially a low-voltage condition and a bad or loose connection, or corrosion on the terminals, can impede flow just enough to throw a false lean signal.3
Turn the chassis-side connector so that it faces you with the large tab at the twelve o'clock position. Note the pin connections; the one on the upper-left is the heater line (HTR), top-right is the power line (VPWR), bottom-left is the signal line (SIG) and bottom-right is the signal-return (RTN) line. With the key on, test the voltage from SIG to VPWR with a digital multimeter; it should read 1.5 volts. With the key off, resistance between the SIG and VPWR lines should be 10,000 ohms. Now, turn to the corresponding pin terminals on the O2 sensor harness itself and test the resistance in ohms; if it's less than 5.0 ohms, you've got a bad O2 sensor -- not a problem with the engine.4
Check your engine's intake tubing and vacuum lines for un-metered air leaks; that means air leaking into the system anywhere past the airbox. Start the engine, then spray a two-second burst of ether starting fluid or brake cleaner around each of the joints in the intake tubing and around all of the vacuum line connections. If you've got an air leak, the engine will draw the fluid in, use it as a fuel and briefly rise in rpm as it burns the starting fluid or brake cleaner.5
Check your fuel pressure fast- and slow-leakdown, using a fuel pressure gauge connected to the Schrader valve on your engine's fuel rail. Cycle the car's key on and off several times to build up fuel pressure. When you turn the key off the last time, the fuel pressure should remain within 5 psi of its highest reading more than a minute after you shut the car down. If pressure drops quickly in less than a minute, you may have either an external or internal leak causing low fuel pressure and a subsequent lean condition.