No heating error code
All modern washing machines monitor the water temperature, and if it does not rise a certain amount within a pre-defined time then it should flag a fault. So, if the heater isn't working, then the program should abort and produce an error code.
On an older washing machine the symptoms should be that it carries on washing indefinitely. And even a few modern washing machines can develop faults that cause the washer to stick mid cycle and wash forever without producing an error code even though they are supposed to. If you suspect something like that, call a repair man. It is not a DIY job.
Machines with no heater
Some machines have no heater, they have to pump in both hot and cold water from outside. You should check your hot water pipe to make sure hot water is getting to your machine, you should be able to feel it getting warmer as the water flows through it.
I can’t feel any hot water through the door while washing my laundry
A common misbelief that there must be something wrong. Not necessarily, because:
• The water is only hot inside the washing machine for a small amount of time during a wash
• Many washing machine doors also have an outer plastic insulating cover so it's sometimes hard to feel any hot water
• Modern washing machines put so little water in the wash that hardly any warm water splashes on the door glass
All in all, not feeling any hot water through the door doesn’t necessarily mean there is something wrong with the heating, especially when the appliance otherwise seems to work OK and the laundry comes out clean thanks to Ariel and your washing machine. So there is no reason to be alarmed too soon!