VR6 low mileage 12-14 mpg and car spec states 29 mpg. After the engine has been running for about 10 mins the car has slow acceleration at low revs in each gear and then it works better at higher revs. I have been told by another VR6 oc member that it could be O2 sensor, vacuum leaks, spark plugs or wires? It feels and sounds like the petrol mix is too rich at low revs so how can I tell if it's O2 sensor causing the low mpg because there were no error messages when the laptop was plugged in at the garage looking for all faults?