Ok, a few weeks ago I started having a check engine light come on and here are the symptoms:

1. Loss of power
2. Loss of Mileage
3. Check Engine light.

Ok, after checking the computer codes I discovered that I had a code 37, left O2 sensor faulty. No problem, I replaced both O2 sensors with new units that are supposedly Factory from BAP import parts. I noticed my overall power improve and I was quite glad. However, this all ended when the symptoms returned in short order. I guess I had a PERCEIVED victory. I decided that the problem was now beyond my ability to diagnose and I took it to my usual mechanic. He's observing on the new O2 sensors that out of the blue the left O2 sensor will suddenly report lean, no voltage. This is usually around the time the light comes on but not always. This lean state also occurs at the same time that the right O2 sensor is showing normal operation. Now, I would assume that a brand new O2 sensor would not act the same as an old bad one, especially considering that the behavior is the same. Also the readings were taken at different points along the sensor path as to rule out problems with the ECU. Could it be possible that something is causing the left bank to go lean and give the ECU and indication of a problem so as to report the code 37? And if so what could cause such a problem. If it helps, the Harmonic balancer recently went out and the pulley as well as both belts needed to be replaced. Including the timing belt cover. Could any activity in this area cause complications elsewhere that may cause these issues. Any suggestions would be appreciated.

- Phatj