2019 M3 Rear Vent blows warm air when low-level cooling is required

[EricElf]

My 2019 Model 3 SR+ seems to have an issue with blowing warm air to the rear vent when it should be cold.

This issue happens when mild cooling is required, for example, outside 76°F and inside temp set to 72°F. The front vents will blow cold, but the rear will blow warm. If air recycle is on, it is even worse. Turning temp down to about 68°F will get cold air coming out of the rear vents.

Searching here and elsewhere, I see lots of complaints about the Tesla A/C systems. Most seem unable to get help from Tesla Service Center.

I saw one workaround. Set the driver temp set as desired (72°F), but separate the front-seat passenger temp and set it low like 68°F. I have not tried this and is not a very good fix anyway if you have a heat-loving front-seat passenger.

Saw a reference to faults with the Temperature Humidity Sensor (THS) ECU. [MODEL 3 SERVICE KIT - HVAC SENSOR(1462944-00-A)] but that is in relation to the car throwing a fault.

Does anyone else have a similar issue and/or know if this is fixable? I wanted to ask before going to SC because of all the related troubles people report.

TIA, Eric

 

[richrootes]

Go into Service Mode and run a thermal test - the results should show if there’s a faulty sensor or actuator

 

[EricElf]

Go into Service Mode and run a thermal test - the results should show if there’s a faulty sensor or actuator

Thanks for the quick reply. Test passed.

Feels to me like a separate rear thermometer and resistive heater to augment rear seat heating that is "messed up" (improperly calabrated) or malfunctioning air blend door actuator, or perhaps just a software bug... All speculation, as I have no idea how the system is configured. Perhaps I could figure out if I searched harder. Hoping someone has been down this road and figure things out.

 

[beachmiles]

This is a 3 year plus bug for all Tesla models with resistive cabin heaters last I checked where the cabin heater is used when you only want cooling. There is a possibility your evaporator temperature sensor is not good but Im pretty sure it's still a software issue.
It gets worse the closer the ambient is to your setpoint or if it's nighttime. The only way to ensure the cabin heater is not used is to set the temp to LO.

cabin heater coming on when only cooling desired

teslamotorsclub.com

 

[EricElf]

This is a 3 year plus bug for all Tesla models with resistive cabin heaters last I checked where the cabin heater is used when you only want cooling. There is a possibility your evaporator temperature sensor is not good but Im pretty sure it's still a software issue.
It gets worse the closer the ambient is to your setpoint or if it's nighttime. The only way to ensure the cabin heater is not used is to set the temp to LO.

cabin heater coming on when only cooling desired

teslamotorsclub.com

Yes, I have seen these. My problem is a bit more nuanced.

Outside 80℉, Inside set to 73℉ with the rear vent on. Front vents blow 65℉ rear blows 75℉.

In this 2019 M3 with resistive heat, it seems there must be a secondary thermometer for the rear. And either a secondary resistive rear heater or an air-mixing door similar to ICE cars (or possibly some interaction with battery temp control). I'm hoping for the simplest explanation: a secondary resistive rear heater controlled by a secondary thermometer (probably a simple thermister) that has gone bad or is not properly calibrated in software.

All thoughts are welcome!