Heater blowing cold/lukewarm

[PrimaVW]

I have a 1999.5 A4 with a facelift HVAC. It blows nice and cold with AC but when I turn the heat on, it blows lukewarm, regardless of how high I turn the temp with the switch. I just had the heater core flushed, and it is hot on both sides. Engine isn't overheating or showing any signs of it being a cooling system issue... so whats the deal?

My mechanic tried to hook it up to diagnostics but it wouldn't respond. I am hearing about a G89 sensor being the culprit? Any other things to check on? Its getting colder at night here and I'd like to have a toasty car sometime before November. Thanks.

[a2thej22]

I've hear of the actual control module going bad, have you tried swapping that out?

[MarcMiller]

i honestly dk much about this but did your mechanic burb the coolant? when my friend did my timing belt i didnt have heat at first until he got all the air bubbles out of the coolant

[QuattroDriven]

Im going threw the same issue. Did you think about changing out the thermostat? Ill keep u posted on what my issue was if it is any help to you! Plan on tackling this job before h20

[YankeeBob]

Interested in what you find out. Having the same problem. Tstat is good. Engine heats up quick and getting hot return water from the heater core. I have ZERO heat.

[a2thej22]

Im going threw the same issue. Did you think about changing out the thermostat? Ill keep u posted on what my issue was if it is any help to you! Plan on tackling this job before h20

I did a thermostat in mine, no dice. Every time i clean out my heater core it works for a day or two then stops.

[CMac019]

Same exact thing that's happening to me, did you end up fixing it? I just flushed all my coolant, changed my thermostat, and both hoses to the heater core are hot.

[WantedBy]

if both heater core hoses are hot, wasnt there a problem with the little flaps getting stuck? i had the same issue i did a drano heater core flush a while ago. but the culprit was my water pump was failing. Notice this the car wouldnt overheat but once i replaced it in a 35 degree weather i found that on of the plastic fins was cracked. hope that helps oh and make sure you burp the system for any air pockets.

[PrimaVW]

Just to answer you folks, I did resolve my issue. I did a temp switch because my temp gauge was dead. This fixed the issue with the gauge but not the heat. My mechanic flushed to heater core but it did not fix the issue. I then replaced the G89 sensor becuase I had no way to diagnose the oem sensor and it was a cheap part to replace. This also did not resolve my issue. I pulled the glovebox and checked my heater flap motors. The blue motor (hot/cold flap) was sluggish, but functioning. I then performed a heater core flush with CLR and burped my system and this fixed the issue. Its been blowing hot for a week now, so we will see what happens. Even so a $5 bottle of CLR even now and then won't kill me. :

[kdf8454]

I pulled the glovebox and checked my heater flap motors. The blue motor (hot/cold flap) was sluggish, but functioning.:

Did you find a DIY for this somewhere? I'm having the same issue - both heater hoses are hot and AC works, but no heat. It's starting to get cold out too

[PrimaVW]

Did you find a DIY for this somewhere? I'm having the same issue - both heater hoses are hot and AC works, but no heat. It's starting to get cold out too

I friend of mine is an Audi tech and he said this issue was commonly caused by a clogged core. He explained it to me that the core itself has flow in and out, but the fins have a build up on them that will cause a lack of heat radiating from the fins into the interior of the car.

The flush is pretty simple, locate your heat core inlet and outlet hoses ( can be found in engine bay, in raintray next to battery.) Carefully remove both hoses, and using two lengths of 3/4" radiator hose, gently run water into outlet side of heater core. Once all coolant is flushed from core, dump in CLR or similar product and let sit for 15-20mins. Flush CLR and repeat as needed. Replace inlet and outlet hoses to heater core, top off coolant and bleed your cooling system of any air bubbles. If this doesn't work, I'd say have a mechanic take a peek since your core is probably beyond this...

Here's a write-up
http://forums.audiworld.com/showthread.php?t=349252

[kdf8454]

thanks for the info. I'll give that a shot this weekend.