Troubleshoot the heater in your 1999 Blazer before temperatures fall. Problems that could arise with the heater are usually fairly simple to diagnose, whether they are a blown fuse or a stuck thermostat. No tools are needed to check your vehicle. Run through a few troubleshooting steps before taking your Blazer in for what could turn out to be an expensive service.
Turn the blower fan to the highest setting with the engine running to check that air is flowing across the heater core. If you do not hear or feel the fan turn on, a fuse could have blown, or the fan may have malfunctioned.
Check the fuse box for any heater fuses that may have blown. The fuse box in a 1999 Blazer is beneath the driver's side dash. Read the instruction diagram on the inside of the fuse box cover to locate the fuse connected to the heating system. If the metal strip is broken or damaged, replace the fuse with another of the exact same amperage rating.
Feel the carpet on the passenger side for any signs of wetness. You may also detect a strong, sweet odor. This could indicate that the heater core is leaking coolant. Have a qualified mechanic investigate further. The heater core may need to be replaced.
Check the radiator for sufficient coolant. The heating system uses a small amount of coolant to warm the vehicle. When the coolant level gets too low, air can get trapped within the system and block the flow of heated air into the vehicle. Start the engine and open the hood. Feel the two heater hoses protruding from the firewall. The firewall is the metal divider between the passenger and engine compartments. Both hoses should be hot. If they are not, the flow of coolant could be blocked.
Ensure that the thermostat has not become stuck in the open position. Start the vehicle when it is cold. Place your hand on the upper radiator hose to feel if coolant is flowing. If you feel coolant flowing through the hose, this could indicate that the thermostat is stuck open. Under normal operation, coolant only begins to flow once the engine has warmed up.
