Heater blowing cold air (all vents)

Hi MSC members,

My wifes beloved Micra, 1996 1.0, 36k miles, has developed it's first fault in 5 years (now that's japanese reliability for you!)

The problem is the heater. When the engine is warmed up I get only cool air coming from the vents (all locations). I have checked the following:

  • The cable from the Hot/Cold controlling the valve is moving the white lever at the bottom of the heater assembly.
  • The lower pipes from the thermostat and into the car are getting nice and hot to touch, the top pipe is just warm. This suggests to me that the thermostat is ok.
  • Heater Fan works all settings.
  • Good Flow on all vents at all speeds.
  • Cool air from all vents, not just centre ones. But marginal 2 degree increase between full hot and full cold.
  • No leaks apparent anywhere.

So, I'm a bit stuck now. My only guess after searching these forums is to strip out the dash and replace the heater matrix, but I'm worried that changing it won't be the thing and it's quite a big job to take on although I am pretty handy with cars. I was thinking of changing the Thermostat just to be sure but thought I'd take advice first.

Any help you can offer is grand. I sense that the MSC, now that I have found it, will be a great place to share my future plans for the car as well, but that's for another day!

PS - I've searched forums and found the following which I don't think apply:

Pollen Filter blocked - N/A, the blower is giving good flow on hot or cold selection and I think this is for A/C cars only?


Club Member
hi nick :grinning: you should be able to test the matrix with a garden hose no probs eh (instead of removing it)
and how high is the gauge going ?
Hi Frank.

Should I disconnect the two pipes in the engine compartment before the front firewall break and insert the hose there? Is this to check the flow and ensure the matrix/valves are not blocked?

As for the gauge, it doesn't have one fitted on my model so I can only guess without removing the Thermostat and checking it in a pot of water.

Many thanks for your reply and further questions.


Club Member
you should have a gauge nick :eek: (all models have one surely ?)
and yes, remove the 2 pipes on the baulkhead and shove the hose into one of them, i would have thought
:blush:Yes well I was thinking of my car which doesn't have a temp gauge, just a warning light for when it gets too hot! Her Micra has a gauge and it was coming up but taking a while longer than I thought it should. Also, I flushed the matrix out as suggested and there is good flow. All back together now, nothing actually changed just pipes squeezed/disconnected and it's lovely and hot again! Wierd, but I think I'll get T'Stat on order just in case!

Thanks for your help anyway!