lean fuel bank 1, lean fuel bank 2

I have a 2001 Sienna with 61,000 miles. It has been well cared for and serviced regularly. Recently the check engine light came on so I took it to my mechanic. It showed codes P0171 and P0174 which is lean fuel bank 1 and lean fuel bank 2. He checked the fuel pressure (47 psi), cleaned the throttle body and blew out the fuel lines. He also checked the mass air flow sensor.

The check engine light has come back on and the van sometimes hesitates and stumbles when accelerating.

Is it possible that both upstream O2 sensors would go bad at the same time? Or is it likely the downstream O2 sensor or the coolant temp sensor?

My mechanic is puzzled.

Anyone have any similar experiences? Any thoughts as to what is wrong with my Sienna?

I am trying to avoid taking it to the dealer as they fried the air bag computer in my other vehicle the last time it was serviced.

It could be many things.

1. Mass Air Flow sensor
2. Oxygen sensors (unlikely since it's rare both get bad at the same time)
3. EGR system (very possible if it stuck in open position)
4. Vacuum hoses leak

There should have many other possibilities. The best approach for you is to get a service manual from a library and check each possible component one by one.

Good luck.

I've seen it MANY times. Either remove and clean the MAF Sensor, or replace it. You can just spray it down with brake cleaner to clean it.

I was looking for information on tail light shorting out when I came on this one. Change your gas filter. A partially clogged filter won't kill your gas pressure, but you will get insufficient flow. The restricted flow will prevent enough gas from reaching the fuel injectors and cause the p0171 error.

Thanks for the tip. I ended up taking it to the dealer. It turned out to be the mass air flow sensor. The dealer replaced the MAF sensor and all is well.