Sour is definitely undertemperature. You can measure with a thermocouple to make sure. The temperature would have to be very low, possibly below 80C to give it a bad sour taste.
Make sure you are following all instructions for the machine regarding warming up and purging (if any is required).
It is possible that it is faulty. If you can get a thermocouple or thermometer just stick a styrofoam cup under the group without a portafilter in place and run for 10 seconds. The temperature should be about 90C or thereabouts possibly slightly less.
I think measurement is the only sure way to troubleshoot. If the temperature is too low then you will need to take it in for repair.