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.

Mass Air Flow (maf) METER

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?

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 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 separate drives to duplicate the two-trip logic. The car has been back with the owner for almost a month.

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.

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

Thx - I replaced MAS and problem was fixed

I was able to fix the problem by replacing MAF sensor. Thx

My 03 lexus rx300 it seems that the transmission is acting up, while I was driving it the transmission or may be the engine jump, while driving the transmission stay stable without changing... any one could give me an idea

It doesn't show any code

kittie305,

. Focus on the Transmission Speed Sensor and the Vehicle Speed Sensor (VSS).

. Drop and clean the transmission pan and magnet, replace the filter, and refill with the proper ATF, if this has not been done in 30,000 miles.

Was replacing the MAS difficult? We are experiencing the same problem in wife's 2000 Rx300. Is it a DIY for a mechanically challenged individual?

MAF Sensors are easy to install.

Would any one know how to fix following the code p0440, p0441, p0446, p1130 on2000 lexus rx 300

Would any one know how to fix following the code p0440, p0441, p0446, p1130 on2000 lexus rx 300
You have a gross leak in the evaporitive emissions canister. The most common fault is a emissions hose that has come apart at the air filter box. Take a look behind it for a hose leading to a vacuum switching valve attached to the air filter box.
Or the fuel cap is very loose.

p0172 on 2000lexus rx300 the car doesn't want to idle right or take the gas... any help