heater blower not running AUDI A3 Sportback (8PA) (09.2004 - 03.2013)

If the heater fan on your Audi A3 Sportback (8PA) doesn't work even after replacing the fan and regulator (controller/resistor), the problem is most likely with the power supply or control signal.

Start your inspection with these three steps:
1. Checking Fuses and Relays
Check the Fuse: Locate the fuse for the heater fan (usually in the fuse box under the hood or at the end of the dashboard). Make sure it's intact and not blown.
Check the Relay: The fan may be powered by a relay. Inspect its contacts for corrosion and ensure it activates when the ignition is turned on.

2. Checking the Fan Connector
Using a multimeter, check the voltage at the connector that connects to the regulator/fan (with the ignition on):
Power Supply (+12V): Check the voltage on the thick power wire (often red or red and yellow) relative to ground (car body). There should be around 12V.
Ground (GND): Check the voltage on the thick brown wire (should be around 0). A poor ground may be the cause.
Control Signal: Check the voltage on the thin control wire. It should change depending on the fan speed set on the climate control unit.
If there is no power (+12V) or ground, look for a break or corrosion in the wiring between the connector and the fuse box (the problem is often caused by corroded connectors in the fuse box under the hood).

3. Climate Control Unit Diagnostics
Computer Diagnostics: Connect a scanner (VCDS or similar) and check the climate control unit (08-HVAC). If the control unit itself is faulty or blocked, it will not send a control signal to the fan.