Pontiac Repair: only slight warm air, heater gauge fluctuation, heater core, coolant

Question
'98 Pontiac Grd Prix.  After car warmed up, only slightly warm air blowing, also, heat gauge goes to near red, drops, later, up again.  Seems engine running OK.  What may be the problem?

Answer
Hi Dave.

I would venture to say that the engine is low on coolant. if that were the case there is more than likely an air pocket trapped in the heater core. as for what's leaking it depends on what engine is in this car. But 8 out of 10 times they are low on coolant maybe the other times is a bad thermostat but then you would at least get some heat.