The Oxygen sensor before the CAT will make your car run shite if its faulty, the sensor after the CAT will not make a difference to the economy or running of the car. Sounds to me like they fitted cheap sensors, i swapped my Pre CAT sensor recently, took 10mins to fit the bosch part that cost £80 been perfect ever since.
You cannot read fault codes from the dash it does not work you do need a fault code reader. I use my old Bluefin Handset to read and clear the codes on my ST170
You can clear codes yourself by simply disconnecting the battery for 20mins when you turn on the car after this your dials will sweep indicating a reset