I just replaced O2 sensor and now check engine light came back on after driving 30 miles. I took to local Aotozone store and they identified as trouble code P0171. Any recommendations for diy repair, what part may be gone bad which sends computer this error code?
I am willing to take challenge to repair myself.

kitch428

Mass Air Flow (maf) METER

this8384

P0171 means the system is running either too rich or too lean. It could be a number of things from the grade of gasoline you used to another faulty part. What led you to replace the O2 sensor and which one did you replace? Was there a code stored prior to the P0171?

CaptainRich

Kitch is absolutely correct. The MAF needs replaced. Toyota part # 22204-22010 (It will cost less purchased from Toyota rather than from Lexus)

I just had the exact same symptoms and code on a 2000 Lexus ES300 with the 1MZ-FE engine. I replaced the MAF sensor with a test part and the code didn't return. I then installed a new MAF and test drove the vehicle 14 miles on two seperate drives to duplicate the two-trip logic. The car has been back with the owner for almost a month.

kitch428

It's just a plug and play device. A 6 pin connector and 2 screws holding it in. However, I have a different way of installing them. I like to pull the ECU-B fuse before I pull it out. This will reset the 'learn' values back to defaults.
Or you could just disconnect the battery negative cable, but then you lose clock, radio, seat memories and such.

...and yes, p0171 is system running lean only. not rich (172) You may be hearing spark knocking due to your injectors not squirting enough fuel. (system lean) The MAF is not producing the proper AIR MASS readings to the ECM. The MAF is the start of the food chain for closed loop.

LBEASON

check mass air flow sensor/ air fuel ratio sensor or intake plenum gasket