Temp gauge dropping when heaters on

[R88TTD]

Hi guys

I started experiencing new problems with MK5 gti
The temperature gauge drops from 90 to 75 then 50 gradually as soon as I turn my heaters on

Now the heaters are blowing hot air as normal
I asked about and some tell me the battery , some tell me the temp sensor and some are telling me the thermostat, water pump was changed about 15k miles ago so that ruled out

No head gasket issues , no over heating

Just wondering if anyone had this issue before Thanks

[ZoliWorks]

If you want to find out if it's the thermostat or not, go on the highway, drive at a constant 130-140kph without the heater and see if the temp drops. If it does, it's the thermostat.

[Octoparrot]

I'd put my money on the thermostat.

[mjmallia]

Thermostat for sure

[Mk5gti]

Hi, did you ever resolve this issue and what was it?

[crookedd]

I had the same issue years ago. Also car never reaching 90 degrees fully in winter or dropping from 90 when constant highway speeds. Issue was thermostat.

[Mk5gti]

So my issue is the needle drops when I turn heaters on
Both thermostats have been done and temp sensors using genuine parts, while driving the needle doesn’t drop, the needle will only drop at idle with heaters on