Mercedes Forum - Mercedes Benz Enthusiast Forums - Engine management light Mercedes E240

Can anyone help? I have recently bought a Mercedes E240 2.6 on a 02 reg. the first day that it arrived the engine management light came on, I contacted the garage that I bought it from (which happens to be approx 240 miles from me) and they told me to take it to a VAT reg garage and get a diagnostics done on it, this I did and the outcome was the garage I bought it from gave the garage I took it to the go ahead to fit a new cat, great! Unfortunately the light came on again, this time a lambda sensor was changed, 40 miles later the light came on. The garage now has recommended that I send the car back to the garage I bought it from as they say the cat they fitted still is not working and they believe that is because the cats fitted a aftermarket cats?
So can anyone shed any light on this? I.e. do the aftermarket cats not really work? Do I need the Mercedes cats fitted?