Avensis t27 heater issue

[ni avensis t27]

Hello everyone, I hope someone can advise me on a problem with my 2014 avensis 2.0 d4d. The heater controls all work fine, car runs at normal temperature (right in the middle) but it seems to alternate between blowing warm and cold air inside the cabin. For example, if I set it at 25 degrees and auto is switched off and Aircon is switched off, fan blower is at the 2nd or 3rd level, (once car is at operating temperature) the cabin is nice and warm, but after 10 or 15 mins lukewarm/cold air starts to come through instead. I have to turn the heating up to 27 or 28 degrees to get it warm again and then turn back down to 25.  The first thing I notice is that the cold air comes from the little side vents on top of the dash on either side, then from the foot well area. The coolant is full and the car doesn't overheat at all. I can't see it being thermostat related but perhaps someone more knowledgeable than me can tell me. Could it have air in the system even though the coolant hasn't been changed recently?It's very frustrating always having to adjust the heating. If anyone can advise me I would greatly appreciate it. Many thanks in advance 

[Bper]

Hi Paul, A few things could be causing this kind of temperature fluctuation. Even if you haven’t changed the coolant recently, it’s possible that air has somehow gotten trapped in the system. Air pockets can disrupt the flow of hot coolant to the heater core, causing the heat to blow inconsistently. You could try bleeding the system to see if that clears it up.

Another possibility is the heater core itself. Over time, mineral deposits or debris can build up, which restricts the heat transfer, especially when the fan is set lower. A coolant flush might help if there’s any blockage affecting how warm the air gets.It could also be related to the blend door or the temperature control flap inside the heater system. These parts direct warm and cool air, and if they’re not moving fully or accurately, it might explain why cold air seems to be coming from certain vents, especially the footwell or side ones. This issue can happen if the actuator that controls the blend door starts to malfunction.

Sometimes, there’s also a coolant temperature sensor involved in regulating the heater, and if that’s sending inaccurate readings, the system may switch to cooler air unexpectedly.

You might also want to double check any climate control settings. Even though you have the "Auto" mode off, some systems will still try to maintain a balance, and cooler air could be mixed in to adjust the temperature. But that should only happen with Auto mode on, so it’s worth a quick look just in case. Starting with a coolant bleed might be your simplest option. Hope this helps.👍

 

[ni avensis t27]

Bper thanks for all the advice, I will have to try and work my way through it. Is there a particular method for bleeding the system?