Heating Not Working? Troubleshooting Tips

Hello!
I'm having a problem with my 1994 Golf 3 TDI. The car just isn't getting warm, even though the engine temperature gauge shows 90°C. Even when I set the ventilation to level 3 and turn the middle knob all the way to the red setting, only lukewarm air comes out! What could be broken? Is there a thermostat that might have failed?
Thanks in advance for your tips!

Regards,
Thomas.

Hi!
Okay, spontaneously, two things come to mind.

1. You have far too little coolant in the system (which would likely be quite bad).

or

2. The Bowden cable for the hot air flap is disconnected or, for some reason, unable to fully actuate the temperature control flap.

Maybe someone else will have a better idea...?!

Take care.
pretzelman01