My 1998 A4 has 105,000 miles. My thermostat was stuck open - evidenced by the lack of heat in passenger cab and the constant cold temperature reading on engine coolant gauge. I just got the thermostat replaced and now have normal readings on the engine temperature gauge. However, I still do not have any heat available in the cab. Anti-freeze levels are OK. How can I determine if the problem is related to the heater control value, vacuum check value, blend door or other?
Follow the heater hose from the thermostat housing to the firewall and locate the return hose. Is the return hose cold? Is the supply hose warm, hot, or cold? If warm or hot follow it to a valve and feel the opposite side of temp. Your system may not be so simple but it should have a hot hose to the heater. If it is hot both in and out the damper door on a plenum is not opening.
Make sure as well that there is no air in the cooling system.