The heater core is the main component for the heating system and bypassing it means disabling the system.
You can try back flushing the heater core. You might have to repeat it a few times and compressed air helps.
Disconnect the heater hose and plug a hose onto one of the heater core pipes. If it is clogged, move the hose to the other pipe. Remove hose. Use compressed air to blow into the pipes, one at a time. Do not blow for too long.
Repeat above until the clogging is cleared. If all fails, the only way to get heat is to replace the heater core.
Dec 20, 2010 at 1:45 PM
