1995 Other Mazda Models Heat light on dashboard

Engine Mechanical problem
1995 Other Mazda Models Engine Size unknown Wheel Drive Type unknown Manual 68000 miles

I have just bought a mazda mx3 import it is classed as a eunos was made in 95 but registered in 98.
I picked the car up on sunday and ran around in it for two days with no problems on the 3rd day the HEAT symbol on the dash has come on and it has not gone off since I have left it standing for 12 hours in the cold but it is still on the car has just been motd last month and has no advisories.I have checked all the fuses and all appear to be ok it is not overheating and the fan is kicking in so puzzled I took it to the local garage who have said it is the catalytic converter?It is booked in for 2 days time to be looked at but I have decided to get it back on the road and sell it have you any ideas on what the fault is as I dont want my pants pulling down on this one.

Thanks