AEG Lavatherm 57560 heat exchanger light?

On my AEG Lavatherm 57560 the heat exchanger light is flashing. I have cleaned the heat exchanger properly and it is clear of dust. I have also put my hand to the back of the machine where the heat excahnger sits and cleaned out any dirt that was in there. I put the machine back on and the fault comes on again. I have tried this 5 times now, even blown the heat exchanger out with a garage airline. Any ideas of where to go from here.