I have been having the same problem recently, the hot water works fine but the heating seems to get too hot and then display E168 once I have reset the system it will work fine for a while that do the same again. It has been getting more frequent now though. I have replaced the heat exchanger recently and the NTC sensor, I have now just replaced the diverter valve. I have tried a new pump and none of these things have cured the problem.
My next step is to replace the PCB on my Baxi 28HE , I had it it installed in 2008 and have a few problems in the past which I have been able to sort through the error code but this one has stumped me so far..