Hi, my girlfriend has a 2015 Chevy Sonic 1.8L with 90K miles. The car has been idling very rough, similar to a misfire, and if the gas pedal isn't pressed during idle it might die. It seems to run OK when driving. The oil level recently went up significantly, and it smells like gas so I think it has fuel in the oil. The oil is still black and not chocolate milk looking so I don't think its coolant. I plugged in a cheap OBDII reader and got the following trouble codes: P0171 system too lean bank 1 , P0131 Heater oxygen sensor 1 bank 1 low voltage. I am thinking it is the upstream oxygen sensor?  It also has some codes related to the thermostat but I think this is separate issue since these code appeared before the rough idle and fluid in the oil: P0597 Thermostat heater control open circuit, P0598 Thermostat heater control system circuit low, P0599 Thermostat heater control system circuit high. I also heard that it could a vacuum leak, but I don't hear any sucking sounds with the hood open. What do you think? Thanks in advance.