2003 Toyota Avalon

Engine Performance problem
2003 Toyota Avalon 6 cyl Automatic 77000 miles

I get a big trouble with my Toyota Avalon XL 2003. One week ago, the Check Engine Light came on. I went to the Auto-zone store to read the error code. They gave me the code P0135 with the description of O2 Sensor bank 1 sensor 1. I bought a new O2 Sensor bank 1 sensor 1 of them about $216. Then I replaced it. The Check Engine Light turned off about one night. After that, the light comes on again. What should I do? I'm confusing with this problem.
thank a lot
February 15, 2009.

P0135 refers to the O2 sensor heater circuit no activity, not necessarily the O2 sensor. The first thing I would check is the relay, located inside the engine room junction box JB.

Feb 24, 2009.
The second mechanic replaced this O2 sensor and he said this is bank 1 sensor 1

I got big problem guy !
I gave 2 mechanics. the 1st mechanic said the oxygen sensor bank 1 sensor 1 for Toyota Avalon 2003 is in the back of the engine. the second mechanic said it's in the front ! I gave the 2nd mechanic,also the cheapest mechanic, he changed the O2 sensor in the front and the check engine light's still on ! He said I need to drive about 4 cycles to turn off the check engine light!
Please tell me which mechanic is right ? I doubt the second mechanic :( I paid $100 to replace the front O2 sensor , it's so easy to unscrew ! I thought the O2 sensor bank 1 sensor 1 is under the engine, that's why I gave my car to auto shop.
and please tell me where exactly the oxygen sensor bank 1 sensor 1 for Toyota Avalon 2003 is ?
I appreciate all of your help . thank a lot
the first mechanic said bank 1 sensor 1 is over there

Feb 27, 2009.
Promise that you won't be mad at me. But the mechanic #1 was right. Bank 1 is the one closer to the fire wall. And mechanic #2 was totally wrong when he said you had to drive 4 cycles to turn the CEL off. It is only 2 trip cycles. He also overcharged you by a bunch. It only tool 0.8 hours of his hourly rate to replace that sensor. Unless his labor rate is 125 per hour.

Anyway, you just mis-spent your money. The original CEL codewas P0135 which means A/F Sensor (O2 sensor) heater circuit (Bank 1 Sensor 1). It has to do with the circuit, not the sensor itself. Here is the diagnostic for P0135

Feb 27, 2009.