P0171 and P0174 mean "fuel mixture too lean" on cylinder bank 1 and bank 2 respectively. Sorry I don't know what causes this. Looking at this same problem on 2000 Ford Taurus. Believe it is going to be in the HEGO (Heated Exhaust Gas Oxygen) Sensors. Bank 1 and 2 detecting too lean a condition. Yuck. Per the 1998 official Toyota shop manual for a Sienna, it could be a bad 1) coolant temp sensor 2) bad HO2S (or Fuel/Air sensors) 3) bad mass airflow sensor 4) restricted fuel somewhere (like the fuel filter in the tank! ouch.) First, check to function of the coolant temp sensor. You can pull it and play wiht your stove and boiling water or just buy a Carchip data collector and download the data to your computer to analyze. Second, before you replace that horribly expensive Mass Airflow Sensor, pull it out and *carefully* spray it clean with brake cleaner or an electronic cleaner of some sort. And do NOT touch any of the itty-bitty wires inside with anything or you WILL be replacing it. Third, if you have HO2S sensors, you can check the resistance of them using an multimeter to see if they are within spec. I forget exactly but it's something low like 9-12 ohms at 68F. If you have Fuel/Air sensors instead, it's dealer diagnostic time. Fourth, if you suspect fuel pressure or fuel flow issues, I recommend dealer diagnostic time too. My best friend got severely burned (2nd and 3rd degree on 50% of his body) messing with fuel and his VW bug when we were in high school. 25 years later, he is still covered in scar tissue and will be forever.
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