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Thanks all. I just replaced A/F sensor in bank 1. It was harder to reach than the one at front. After the work done, same thing happened again after a couple of days. Before I replaced the sensor in bank 1, I did clean up the air flow sensor which Lexlogic mentioned. Now I just noticed that there is a broken part on the rubber seal along the air filter, which was replaced not to long ago. Is that possible the broken part on the air filter cause too much unstable air flow intake? Any way, I’m going to replace it and see what is going on.

steviej, Thanks for your fast response. I may try to replace the one in rear bank by myself. Is that hard to locate and reach for that sensor? Meanwhile, I'm still wondering what the guy said. Is code P0171 mean defective air fuel sensor? Or, some thing else? Thanks

I have a 2000 EX300. The car got a check engine/trac off light on problem recently. I took it to auto zone for reading the code. The code showed P0171 system too lean bank 1. The guy who did test for me told me that the problem is the defective O2 sensor. He also showed me the sensor which is just right in front of engine. I bought the Bosch universal O2 sensor from oxygensensorwholesale.com which costs me $68. The sensor looks exactly same as original one. It comes with 2 black wires, 1 blue wire and one white wire, and also comes with a kit for making connection with the old plug. After replaced new sensor I disconnected the battery for 10 minutes to reset the code. It seems to be fine when I restart the engine because check engine and trac off light went away. It is less than 12 hours. The check engine and trac off light comes back on again. I really don’t know what is going on now. :( Maybe I should buy OEM sensor? Maybe I replaced wrong sensor? Or maybe some problem else involved? I remember the reprehensive did mention me that there two O2 sensors before catalyst when I placed order. Should I replace both of them? I really need help to figure out the difficult. I appreciate any info. Thanks