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..
I have now replaced the PCB and the boiler is working fine. Did not have to do anything with the gas supply just switched the power off installed the new PCB and switched back on. An error code did come up E133 which is gas supply fault but i just reset the system and everything is working fine. No more E168 showing and I picked one up for £40 new...Happy
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