A4 2.0 BPW: Hardly any heating

Hello everyone,
My A4 refuses to heat properly at the beginning of winter. I've owned the car for 4 years and haven't experienced these problems before.
I couldn't find any error codes, and I suspect there's a problem with the auxiliary heater. It's known that BPW systems don't heat up very quickly even with gentle driving, but that's what the auxiliary heater is supposed to help with.
"The timing belt and water pump have been replaced, the thermostat has been replaced, the cooling system has been flushed (it was already clean), and the system has been bled. The thermostat is working; after about 25 km, the radiator is slowly warming up to approximately -2°C. The vehicle has a new battery."
Can errors in the PTC heater be diagnosed using VCDS?
My wiring diagram (from Etzold) doesn't indicate where any fuses might be located. It should be more than 25A, right?
Perhaps someone can help me with my first problem here .
Thank you, Matthias.

Hello!

First, you should check the coolant temperature using VCDS while driving, if possible. Alternatively, you can simply feel the hoses leading to the heater core to see if both are getting properly hot.
"In my 4.0 TDI, the heating system starts to get noticeably warm after I've driven about 5 km. This happens after roughly 5 minutes, mostly in city traffic, without any traffic light stops."
The temperature gauge in the dashboard is showing about a third. That should be around 50-60°C. That seems about normal.
Then let's continue...

I don't have a PTC heater, and I'm not familiar with them.
However, you should be able to measure the current draw using a clamp meter connected to the battery cable to see if anything is consuming a large amount of power.

gundi wrote:

My wiring diagram (from Etzold) doesn't provide information about where any fuses might be located. It should be more than 25A, right?

The fuses for these components are located on/in the fuse box in the engine compartment. In the Passat, they are located directly next to the battery, and likely in a similar location in the A4.

PTCs are typically connected in two stages, either 1/3 or 2/3. Since the total power is 1kW, this would be approximately 300W and 700W respectively. At 12V, this would likely require fuses of 30A and 80A, or a combined 100A. At least the larger ones use screw-type fuses located on the front of the fuse box.

The function of the PTC (Positive Temperature Coefficient) heater can be read out from the engine control unit (ECU) in the Passat. To do this, you need to go through the measured values and check them yourself.

Greetings.
Guste.

Please check the coolant temperature sensor to see if it's providing plausible values. You can also use VCDS to read the "deactivation conditions." In my case, the "MSTG" software setting, bit 06, was incorrectly configured. I had problems with this part for 1 1/2 years, but now it's working again. You can also test the function of the PTC (Positive Temperature Coefficient) heater using the actuator diagnosis function.