Vauxhall Vivaro heater fan turns off after about 50 minutes

every morning having to top up with coolant, not much but?
after 50-60 minutes of driving the engine warning light comes on ( stop engine failure hazard) the heater switches off any hot air, (it turns into almost cold air,)

Done this many times.
check water oil (no problem)
If i carry on driving engine failure signs comes on goes off comes on goes off no time , no rythem or rhyme.
All other items cruise control etc don't work again till the engine failure light goes away for 5-10 minutes and comes back.
No heater till it cools down for hours.
The fans stays running when I stop for quite a while which never happened before.
Partner ill and she is always cold so heater not working is a problem.
when i start the heater only seem to work on passenger side not the driver, and its never super duper hot.
My mechnic say he does not know what to do.
Your help will be appreciated.
Thanks Ravi.