Is the sensor reading the fan speed located in the fan itself?

My left fan is working full speed (goes down to normal speed for few secs) . After lots of troubleshooting, I deduced that the computer is not able to read the proper RPM of the left fan (very low 0-200 RPM), hence it operates the fan at the highest speed as a fail-safe precaution.

Now I am wondering, if this reading failure is stemming from a logic board issue or fan issue.

So where is the sensor located? Is it in the fan itself or on the logic board?

I ran AHT and it returned 4MOT/4/40000002 left side -0

I am asking to check if I am screwed or not :), and to decide whether to buy a new fan !!

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UPDATE: Disconnecting the fan from the logic board and reconnecting it again solved the issue. The fan has been normal for a week now.

I hope no one faces this issue, but if you do, try this before buying a new fan !!