P0138 - Meaning in laymans terms??

Just rec'd my U480 OBD2 reader from talktomycar today - very good piece of kit (got this one as girlfriend has a new Tigra and apparantly it has this CAN software!?). Anyway, plugged it in and got P0138 - HO2S Circuit High Voltage Bank 1 Sensor 2. Can anybody help as to what this actually means?? Is it a lambda sensor at fault or more wiring (due to voltage mention?). Many thanks in advance Dave

A problem with your downstream oxygen sensor, or the sensor behind the cat at the rear of the car. I would firstly remove it, give it a good clean with a wire brush and soak it with some carb cleaner shake it and leave it to dry for 10 minutes. stick it back in start the car it may run rough for a few minutes but normally it does if you clean the front one, shut down the engine clear the codes and then run it for a few days and just re plug in your code reader if it does re appear buy a new sensor.

Many thanks! I will have a go at cleaning them both then - see what happens!