code readers on 2000 Nissan Altima

I used a diagnostic code reader and came up with two codes, P1126 and P0138, can you tell me what stand for?

Asked by for the 2000 Nissan Altima
The trouble code data is listed below. I would start by replacing the engine coolant thermostat, and checking the O2 sensor heater element for the sensor listed. (Front Sensor, Bank 2)
Trouble Code Conditions:
DTC P0115 not set, engine started, IAT sensor more than 14°F and ECT sensor from 14-149°F at engine startup, engine runtime over 10 minutes, and the PCM detected that the engine temperature did not reach at least 176°F under these conditions.
Possible Causes:
* Inspect for low coolant level
* Inspect for incorrect coolant mixture
* Check the operation of the thermostat (it may be stuck open)

Trouble Code: P0138 2.4L L4
HO2S-12 (Bank 1 Sensor 2) Circuit High Input (Rich Shift)
Trouble Code Conditions:
Engine started, vehicle driven to a speed over 20 mph for 1 minute, IAT sensor from 14-122°F, fuel level over 25%, and the PCM detected the HO2S signal did not reach the maximum voltage level of 0.60v, or the signal remained at more than 600 mv in the test.
Possible Causes:
* Fuel pressure regulator leaking or fuel injectors leaking
* HO2S may be contaminated or it has failed
* HO2S heater is damaged or has failed
* PCM has failed
* TSB 01-016 (9/01) contains information related to this code