The O2 sensor varies the supplied reference voltage based on oxygen content in the exhaust. It is capable of varying from .1 to .9 volts, .1 indicating lean exhaust and .9 indicating rich exhaust.
A condensed explanation of fuel trims: If the o2 sensor indicates that the oxygen voltage reading is .9 volts or high, the PCM interprets this as a rich condition in the exhaust and as a result decreases the amount of fuel entering the engine by shortening injector "on time". The STFT (short term fuel trims) would reflect this change. The opposite would occur when the PCM sees a lean condition. The PCM would add fuel which would be indicated by a single digit positive STFT reading. On a normal engine the front o2 sensors switch rapidly back and forth two or three times per second and the STFT would shift positive and negative single digits to add and remove fuel to compensate at a similar rate.
This little "dance" goes on to keep the air/fuel ratio at it's optimal level.
Short term fuel trims or STFT reflect immediate changes in fuel injector "on-time" while long term fuel trims or LTFT reflect changes in fuel over a longer period of time. If your STFT or LTFT readings are in the positive double digits (ten or above), this indicates the fuel system has been adding an abnormal amount of fuel than is necessary to keep the proper air/fuel ratio. It may be overcompentsating for a vacuum leak or a stuck lean o2 sensor, etc. The opposite would be true if the fuel trim readings are in the negative double digits. It would indicate that the fuel system has been taking away excessive amounts of fuel, perhaps to compensate for leaking injectors or a stuck rich o2 sensor, etc. So when experiencing o2 related issues, reading your fuel trims can indicate what the PCM has been doing over the long term and short term with regard to fuel.
Your rear O2 sensor code is for low voltage input. The internal heater element may have failed or you could have a short to ground. You can test both O2's out of the car, on a bench with a torch.
For continuously lean O2 sensor readings:
1. Check sensor output wire for possible grounding. A ground will cause a false lean signal.
2. Check the MAF sensor for proper vacuum to voltage output. A high vacuum signal will cause a lean ecu reaction. (Don't forget to check manifold vacuum first!)
3. Clogged injectors can cause a false lean condition. A cleaning may solve the problem.
4. Water contamination will cause a lean condition.
5. Low fuel pressure will cause lean conditions at any rpm or load range. Be sure to check pressure at all driving modes.
6. Exhaust leaks, especially near the sensor can pull in air and cause a false lean reading.
7. Check for proper air injection system operation. The air pump should not direct air to the exhaust ports during closed loop operation.
For continuously rich O2 sensor readings:
1. Check the fuel pressure. High readings will cause rich conditions.
2. Leaking injector(s) will cause rich exhaust.
A leak down test and/or a power balance test can usually reveal the leaker.
3. A contaminated or malfunctioning canister purge system can very easily put uncontrollable amounts of fuel into the intake manifold. Simply disconnecting the vapor hose can reveal this as your problem system.
4. Check vacuum to voltage readings at the MAF sensor. A low MAF output will cause a rich ecu reaction. (Don't forget vacuum readings again!)
5. A false tps signal can cause the system to go rich if the Ecu sees a high tps output. Check tps readings at idle and for a smooth rise to wide open throttle.