I had an alternator changed last week. Two days later I received a "STOP" coolant warning so I got my car recovered to the garage. The coolant was completely empty and we found that the coolant hose at the bottom of the engine was disconnected, not sure for how long. Garage filled the coolant up and run a carbon mono-oxide test on the coolant tank. The result was that the liquid in the apparatus turned green, which apparently, was indicative of the presence of carbon in the coolant, in other words a head gasket failure. There’s no oil present in the coolant pot and even though the coolant level has dropped by about 30% in 24hours, there are no other visible signs and the engine runs very smoothly. I spoke to two different garages and they both express doubt that it is a case of HGF. Now waiting to have it booked in for second opinion.

What do you think? Thanks!