Anomalyx

so i took my 2000 v6 stang conv. to a shop last week to get a few minor problems fixed, and half a day after getting it back, engine light came on. P0155 02 Sensor Heater Circuit Malfunction. Took it back. They re-did the 02 sensor (they had replaced 2 02 sensors that were switching extremely slowly when i brought it in the first time). Drive for another half a day, light comes back on, same error code. Since school starts the day after tomorrow for me, i dont really have the time to let my car sit in the shop. From this description, can someone tell what my problem is, or most likely could be? It seems to me like it's not the 02 sensor... unless 2 bad 02 sensors got installed in a row

Unrelated problem if anyone feels generous with their time:
Excess fuel consumption. Only 14 mpg city, bit over half of what i expect. However, I get relatively normal gas mileage on the freeway... almost 30... odd?

Thanks