My daughter’s 2002 Honda Civic is overheating unless you drive over 60 mph. The heater is also not working - it blows air but the air is cold. It is full of coolant - we checked. The thermostat was replaced about 2 months ago. This issue started today. Any ideas?
It doesn’t dismiss another bad thermostat nor issues with cooling system obstruction. Take it back and have it checked out unless you feel confident checking the thermostat yourself. Do it ASAP as even mild overheating frequently can cause the head gasket to go. I would not drive it anymore then necessary…to get it to the nearest mechanic you can trust…
Since it does not overheat when you drive on the highway, it could be your cooling fan not coming on when needed. At highway speed there is enough air flow through the radiator and you don’t need the fan.
The heater not working could be that the air control door, actuated by the lever that selects defrost, heat and vent, is stuck in the vent position.
Have both of these items cheked out by a competent mechanic. Don’t go to a tire or muffler shop, since these businesses don’t specialize in odd problems.
Check that both electric fans are coming on
The heater core may be plugged
Are the hoses that go through the firewall from the engine compartment to the heater both hot to the touch? That might provide a clue to the indoor heat problem.
Also, if this happened right after a thermostat replacement, there may be air in the cooling system, preventing the coolant from flowing freely.