The 87 octane has absolutely nothing to do with the fault codes. Unless your owners manual specifically says to use anything other than 87 octane, you should not be using the higher octane anyway without a specific reason to do so.
For the P0161 code, consult the troubleshooting section of the service manual or CD. The fault may be due to a defective O2 sensor, defective wiring/connections, or a fault in the PCM. Just replacing the sensor based on the code might or might not fix it since no troubleshooting will have occurred.
For the 171 & 174 codes, see one of the many recent threads on those codes for advice.
Steve
