Coolant temperature and fan issue

[BLF623]

Any suggestions would be appreciated as I’m all out of ideas. I have a mk3 1.8 Petrol Mondeo. It developed an intermittent fault were the engine temperature would fluctuate and also take a long time to warm up. I have now changed the thermostat and it gets up to and holds temperature very fast. The issue now is the fan will kick in at approx 98oc but will then not turn off. I have plugged it into a reader and even with the fan running the temperature remains at a steady 100oc. I believe the fan is running at its lower speed during this time. If I turn the a/c onto demist when the engine isn’t at full temperature the fan will come on in high speed and will turn off when a/c is turned off. I have checked relevant fuses and I have also changed the coolant temperature sensor. My code reader finds no faults but the dash diagnostics now show FTC E 199.  I am well and truly stumped. Any suggestions would be greatly appreciated. 

[BLF623]

Apologies fault code is DTC E199. I also forgot to mention I fitted an aftermarket thermostat not sure if this could be the issue.