An engine lamp lit and a P0174 code retrieved is in fact a lean bank two concern. Bank 2 on the Freestar is the side of the engine that is closest to the radiator or front of the car. There are several concerns that can trigger this code. The reason the engine lamp comes on is because the oxygen sensor on that bank has sensed more oxygen then expected. When this happens the computer will increase the fuel injector pulse width, in turn adding fuel to compensate.
A leak in the intake system will generate a lean code. What we need to know is whether there are any pending codes. Many times a bank # 1 code will be pending or be close to being set. If this is the case the intake leak could be anywhere from the mass air flow sensor to the cylinder heads.
If P0174 is the only code and there are no other pending codes or high fuel trims (*will explain) then you need to look for air leaks that will only affect that bank. I would not expect a bad oxygen sensor without other codes indicating so. The oxygen sensor is doing exactly what it's supposed to do. An exhaust leak near the bank two oxygen sensor can cause this concern. As the exhaust pulses, it can draw in fresh air which will be read by the sensor. Also, a fuel injector not spraying the proper amount of fuel will cause a lean code however this would lead to a rough running or missing engine.
The best way to diagnose this concern is to have a scan tool hooked up, monitor the fuel trims and spray an accelerant such as 2+2 carb cleaner around the air intake system. If there is a leak, the fuel trims will go negative. (*fuel trims are used by the computer to monitor and adapt to conditons and for diagnostic purposes)
A visual inspection may show your concern but in most cases a scan tool that monitors sensors will be needed. Basic diagnosis will be between 1.0 - 1.5 Hrs. x the shops hourly rate. This should allow enough time to properly diagnose your vehicles concern. Most shops charge an average of $99.00 per shop hour.
Good luck, if you can't give four thumbs up, please ask for more information and I'll try my best to help. Have a great day!
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