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Have a 2005 LL Bean. Codes it is giving on CEL are P0171B and P0174B.
My mechanic thinks my Variable Valve Timing has a problem. I think the new AF/O2 sensor he put in a couple months ago is malfunctioning. Prior to these codes we were also getting "Lean Knock Codes" for both banks (sorry, mechanic never would tell me the numbers). Anyone have any input??

A little history here: car purchased last year 82,000mi, 3.0R, had the P0420 problem (no, I don't need advice on P0420). Has had the P0420 intermittently ever since. Has had new cat installed on right bank and new O2 sensor on right bank (all the 0420 codes have been right bank). In September the CEL came on and it was a bad AF sensor on the right bank. Replaced it. Car ran great until about two weeks ago, no more P0420, gas mileage went up, car actually accelerated again, seemed like our 0420 problem was solved! Now, CEL came on again, no more 0420, now lean knock codes started showing up on both banks, we aren't getting those codes now, they seemed to have stoped after we got rid of a K&N filter and cleaned up a fouled MAF sensor, and now I have P0171B and P0174B (right bank AF trim) and no more knock codes. Does anyone have a guess what the problem might be? Again, I think the new O2 sensor is a logical place to start, my mechanic is trying to send me to a dealership, and I can't afford them, and all they are gonna do is send me back to the mechanic (I think) after they gouge me for bunch more money, and tell me the mechanic needs to replace his bad O2 sensor.... Mechanic says VVT is stuck (I don't buy that) Any input here???, no, not about 0420, but what is 0171 and 0174???
 

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P0171 and P0174 are "System too lean (Bank 1)" "System too lean (Bank 2)" respectively.

Given that the codes are for both banks, look for something that's common to both sides. This includes the intake manifold area (e.g. look for disconnected hoses, cracks), exhaust (leaks), fuel supply (poor fuel pressure/delivery), engine coolant temperature (incorrect signal to the ECM), and MAF (not reading air flow or air temperature correctly). A live data scanner might be helpful to monitor related parameters real time. The dealer will have the Subaru Select Monitor for this; other shops might have similar tools. The trouble codes provide some insight into what is causing a symptom but they don't necessarily identify which component (or components) is failing. The idea behind scanning is to see what's actually happening among a number of related data streams; this enable the mechanic to narrow down the possibilities. (See, as an example of this technique, http://www.subaruoutback.org/forums/66-problems-maintenance/49537-p0420-diag.html)
 

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Too lean?

I'll assume for the moment your mechanic is smart but just in case I always check the "ya, duh" items.

Air cleaner, air box, MAF, throttle body... a good dose of sea foam in the tank and some in the intake could not hurt to clean everything out. I would highly suspect an air system problem.

Start with air system and all related parts. If they all check out check the fuel system.

If you have duel exhaust, one bad O2 will not give the code on both banks so that would suggest the new parts are working (does this car have a "Y" pipe?). Sometimes new parts do fail but experience has shown me not to assume that from the start.
 
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