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I have a very strange issue with heat on my 2003 Honda Civic EX.  After the car is warmed up, the heating system only blows warm air while I'm driving over 40 mph.  If I am in park or idling at a stop light, cold air blows out of the vents.  The dealership checked the whole cooling system and couldn't find any leaks.  They also replaced the thermostat, but the problem continues.  So now I'm out over $200 and the issue is still occurring.

Anyone have any idea what the problem might be?

Thanks for your help!  

did you/ they write on the service order...   customer complaint...heater blows only warm air at speed...  blows only cold air at idle...

if they did not fix the problem... they should not have charged you.


i wonder if there are any vacuum leaks to the heater / ac controls...  that would also limit the amount of heat...


are you sure they changed the thermostat??????


i will be back .... i gone to look for more info...  

I have had my 08 Honda Civic to the dealer now six times to have them fix this issue since November 2009.  I had excellent heat from the time I bought the car 3/09 until one cold night in November.  They have not been able to fix the problem either.  They said they replaced the thermostat, and a modulator, they ran it against another 08 Honda Civic and said mine was out performing the other as far as heat temp... that's a scary thought.

So now the dealer has it all this week and I have not heard yet what they've done.  They said Honda was going to tell them what they should try next.  They said nothing is coming up on the diognostics....  I got the car with 19,000 and it has 28,000 now...so it's still under warranty... but the repair department doesn't make anything on warranty work so they don't even try.

I have to drive about 30 miles or get the RPM's up around 60 MPH and get the temp gadge to stay at Max and NOT turn the fan speed up past medium to maintain warm air.  I live in MN.... HEAT is kind of needed the majority of the year!  Good luck with your 03 but know that yes others are having an issue & haven't gotten it resolved yet.  

time to inspect hose temps with a hand held temp gun...

the engine should come up to full operating temps in 3 or 4 minutes...


take the hoses off the heater core and check for restriction through the core......

replace the heater core....  it might have gotten clogged..   or had corrosion problems...     why do i say this..

aluminum when exposed to old antifreeze can be dissolved .. and redeposited else ware... these deposits are aluminum oxide.. a ceramic and a good insulator..  

is there any chance that the heater core could be blocked by debris that has come down from the air intake...

or the cabin filter is dirty???

finding out why the engine takes a long time to warm up is the first thing...  and may cure the problem...

i wonder if the water pump is working properly...   the impeller blades not broken off or eaten away....