Hotpoint FZ175G false warm temp error

Hi all,
I have a Hotpoint FZA175G frost-free upright freezer, around 3 years old, and the red Warm Freezer Temp light occasionally comes on. I have checked the temp inside the freezer using a thermometer and it is still cold, around -15 C.
There was a small buildup of ice at the bottom of the freezer which I have cleared, but the problem persists. There is also strange knocking, banging and crackling noises from the freezer occasionally - not sure if this has any relevance.
Does anyone know where the temp sensor is in this freezer, and if it is malfunctioning, and what would cause it?

Anyone got any ideas on this?

red light on means compressor fault

before you condem the appliance or get an engineer to look at it, please get your food stored in a working freezer and switch of your hotpoint off for 24hrs with the door left open, then retry your freezer as somtimes the evaporator gets blocked with ice and the defrost system cannot clear it.

Surely it can't be a compressor fault if it is still cooling? It is currently -15°C. There is no buildup of ice anywhere in the freezer. What else can cause the red light to come on occasionally? A faulty sensor or faulty wiring?

short of gas best to get engineer to look at it