I came across with a FB post couple of months ago that someone took his expensive car with MAF sensor issue to a mechanic workshop. He ended up being charged for a scan fee but the mechanic in the workshop didnt fix the issue as the car was still idling rough when he went to pick up his car.
Just like any other sensors in a car, MAF sensor is also one of the electronic devices in today's modern and complicated vehicle's engine management system. The device needs to be checked and tested in order to confirm that it is faulty, not by assumption. Most but not all good mechanics can do the job and understand about the principle of how each device works. But for issue like this, the one that car owners really want to see is - an Auto Electrician. But yea, it is hard to tell and differentiate when to pop in to see an auto electrician or a mechanic when something went wrong on your car so people tend to just go back to the usual workshop that you always get your vehicle serviced and warranted.
Back to MAF topic, petrol engine needs air + fuel for its combustion so that it can rotate. The amount of air going into the internal combustion chamber needs to be measured so that the correct amount of fuel can be injected into the chamber at the sametime, by the ECU. Hence, the MAF comes in place.
There are a few different types of MAF in the market, the vane meter, karman vortex, hot wire and etc. The hot wire is most commonly used in new modern cars as it can be easily installed in between the air intake system, no airflow restriction issue, cheaper and more durable. It has a platinum wire in the middle of the sensor, heated up at all times when engine is running as current flow thru it, and increase it's resistance. When vehicle is on the move and cool air flowing in to engine bay going thru the air intake system, the wire will be cooled down and reduces the resistance. The changes in current to maintain the same resistance on the sensor reflects the amount of air flowing into the engine. The electronic circuit on the sensor will then deliver this changes (signal) to the ECU for reading.
So what will happen when this sensor is faulty? Most commonly you will see your engine running rough when idling, engine stall not long after starting and hesitation when accelerating. MAF should be removed for inspection if you do get issues like these. If grease or oil residue is found covering the hot wire inside MAF, a MAF cleaner or electronic cleaner can be used to rectify the problem. Otherwise if the hot wire is broken or other electronic parts like resistors around the hot wire is burnt or overheated, then a new replacement unit will be needed.