The check engine light has been on for about four years. It came on shortly after the first DEQ inspection then off for a week. On for a week, off for a week and then it just stayed on. I did a couple of road trips and once i was on the highway and cruising the light would go off and and then come back on shortly after coming back home. The car was totally fine other than the light and nothing seemed to be wrong (although the water pump decide to die 3 years ago, it didn't effect the light) so i never really worried about it until the next DEQ inspection came up. I took it in and they said it was running lean, from the codes. They changed the O2 sensors and I the light went off and I passed DEQ. Driving home from the DEQ, the damn light came on again and stayed on until now, stuck needing to pass the DEQ. The only time in the last two years the light went out was when the car wouldn't start one morning (it would turn over but not catch) and i replaced the fuel filter. The light went out for about two weeks and then came back on for a year.

Anyway, the codes that came up when I tested the computer just now are P0153 (O2 sensor circuit slow response [bank2 sensor1]) and P1151 (lack of HO2S21 switches - sensor indicates lean)

Any suggestions on where to start?

 

I also cleaned the throttle body, mass airflow sensor, replaced the fuel filter and performed an engine flush. The car ran better but the same two codes came back after clearing them with the reader. I'm going to purchase an O2 sensor and replace Bank2 Sensor1. I have the 2.5L V6 duratek engine in the contour LX model. Does anyone know where that sensor is located. My guess is on the manifold before the catalytic, but how do i tell it apart from the bank 1 sensor?