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My son has a 2004 Honda Accord EX 2.4L. The interior heat stopped working and he can't defrost his windows in the morning.

Here are the things I've done to fix the problem:

  1. Replaced the thermostat
  2. Replaced the water pump
  3. Replaced the radiator (it was old and in bad condition)
  4. Back flushed the heater core -- it flushed out with no issues
  5. Back flushed the entire cooling system -- no apparent issues

Having done all of this, there is still no heat coming from the interior vents in his car. After running the car hard for a while, the top radiator hose gets hot, but the bottom does not. This makes me think the thermostat is not working, but I tested it in boiling water and it does open.

Any help would be appreciated.

  • Does the engine is heat up correctly? I'd expect the valve cover and radiator to get warm when the engine runs, and the temperature gauge in the instrument panel should read near its midpoint. – David Nov 23 '18 at 22:32
  • Welcome to Motor Vehicle Maintenance & Repair! I'd suggest the thermostat may not be opening at the correct temp. Boiling is way hotter than where it should open at. You could retest it, keeping control of the water temperature checking it with a thermometer to see when it actually opens. It should open around 190ish ... – Pᴀᴜʟsᴛᴇʀ2 Nov 24 '18 at 1:20
  • I was thinking it’s a stuck air blend door in the heater. I’ll flesh it out in an Answer if the OP reports the engine is actually at the correct temperature. – David Nov 24 '18 at 1:23
  • David: Thanks for the input. The engine does get warm and the temp gauge reads at midpoint. I have not checked the air blend door in the heater and I'm not even sure where that is, but will research that more. – G. Mill Nov 25 '18 at 19:19
  • Paulster: I did checked the thermostat with a thermometer and it opened at about 195. – G. Mill Nov 25 '18 at 19:20
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Within the heater assembly is a small door that routes incoming air either directly into the car's interior, or through the heater core to be heated before entering the car's interior, or something in between to allow airflow through both paths. It's usually called the "blend" door, and it's likely stuck in the "no heat" position.

I'm not familiar with the '04 Accord EX, and don't know how the blend door is controlled. There may be a rotary knob on the dashboard which connects to the door via mechanical linkage or a bowden cable. A more complicated variant uses a vacuum-operated actuator at the blend door to move the door; the driver uses pushbuttons or a rotary control on the dash to choose where vaccum is routed, and thereby open or close the door and change the air path. Another possibility: the car's heating system may automatically regulate the air blend by sensing the car’s interior temperature, and adjusting the blend door position using an electric actuator at the door.

You'll need to locate the blend door, confirm that it’s not moving, and troubleshoot the cause.

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