marmer

It gets lukewarm, OK when it's 60 degrees out, not so when it's 30 degrees. I just flushed the heater core as suggested in the tech article, ran 5 gallons of water through in both directions, no obvious clog. Temp gauge works as normal, car isn't running unusually hot or cold. No error code in climate control head unit; airflow to ducts seems normal. What else should I be looking for? What would the symptoms of a bad thermostat be?

Thanks

Casablanca Avant

If the t-stat has not been replaced might not be a bad idea to replace.

A4SilverT

Check if the temps from the various sensors are roughly the same as what a thermometer reads. In short, if any of your temp sensors are malfunctioning, the climate control could think that the interior is warmer than it actually is.

Also, the fan that brings interior air into the interior temperature sensor (behind the grill between the radio and climate control panel) might be malfunctioning. That's a common problem.

Did you investigate the possibility that the hoses between the heater core and the engine are blocked, that is, between the point where you disconnected them from the firewall and the engine?

marmer

I did run the codes, got 0.0, which I think means no malfunction detected. I should check the little dash fan. Can I feel/hear air movement if it's working?

About the heater hoses from the engine: I was actually expecting coolant to come out of them when I disconnected them, but none did. Only a very little bit out of the heater core itself. Is this to be expected, or maybe symptom of clogs?

Thanks,

A4SilverT

In reverse order:

Because the hoses are at the top of the heater core and there's no pressure, you won't loose much coolant. For the engine hoses, try running a drain snake through them, or an old speedometer cable, or even a piece of 14 ga wire. But just through the hoses, not into the block.

I can't hear my fan. Try putting a light piece of paper up against the grill and see if the suction will hold it in place. It should fall off when you turn the ignition off. Or try some smoke near the grill and see if it goes in.

In regard to the diagnostics, I meant for you to get the temps that the various sensors are reading. Compare the interior sensor reading with a reading you get from a thermometer inside the car. Compare the fresh air intake sensor reading with a thermometer reading of the outside temp. See if those sensor readings make sense.

Failure of the sun sensor on the top of the dash could also contribute to the problem. But it would have to read "sunlight" when no sunlight is present to do so, and I just don't know if a failed sensor would read sunlight or no sunlight. When a light sensor for house lights fails, it leaves the lights on, i.e. it allows current to pass through, thinking that there is no light. But I don't know how that applies to Audi's sun sensor and what signal it's sending to the climate control.