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I have a 2000 Avalon with 83k. About 8 months ago, the check engine light came on. Also, the power is not what it used to be. Auto parts store ran the diagnostics and came up with P0171. Based on recommendations, I cleaned the MAF sensor. No change. I then bought my own diagnostic unit and it had the same P0171. It also had an O2 test which I ran. It showed a downstream O2 sensor problem. Yesterday, I replaced that O2 sensor with new OEM part. The power wasn't improved and the O2 diagnostic issue remained. Anyone have any ideas what to try next?
 

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Could be:

Low fuel pressure.
Engine coolant sensor.
Leaks in intake or exhaust system.
Injector blockage.
MAF.

If A/F sensor typically it is one of the first ones. The A/F sensor have their own codes.

If no obvious leaks might still be the MAF.

Suggest visiting the AutoZone website and looking in the vehicle repair guides. The site often has specific tests for components. If Avalon is not listed try the Camry with V6 option as same engine, almost the same car.
 
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