So I noticed that when A/C was on high speed, the force of the air would decrease over a 15-20 min period, this was when I was demanding A/C on the Low setting (AC is cold) Even though the blower control readout was showing it to be on high. I found that switching the system off and then back on resolved the issue for another 15-20 min. I replaced the resistor but this has not solved the issue. No funny noises from the blower motor, runs great other than the speed reduces when on high after a period of time. Not noticed it reducing driving in the city, more so on the highway…what could the problem be? Thanks in advance
Oh and after reading another post about this, I should add that the fan speed reduces even when not demanding A/C ?
Sounds like your blower motor is failing.
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Agreed, when the blower motor gets hot, it slows down.
If your vehicle uses normal resistors to limit the motor speed then I would suspect the blower relay contacts are heating up and causing a voltage drop, which would change the speed of the motor. If the relay is getting real warm or hot after a bit that would be a sure sign of trouble.
The evaporator core is icing, there is a thermo probe in the evaporator, the computer should cycle the system to keep the evaporator from going below freezing.
In stop and go traffic the A/C system generally doesn’t perform well enough to get cold enough to freeze.
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The OP states the blower also slows with the AC off.
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I suspect the OP is only adjusting the temperature setting and not switching off the compressor.
Especially if it’s a manual control another thing to check is the actual fan speed control switch.
Don’t know your car’s age but it was a common problem on early Hyundai’s
Replacement parts were available from Koraps.com
Thanks fir your reply, so how can I resolve the issue, how do I stop the icing?
Get your A/C system tested and serviced.
Great thanks, job for next spring 
. Happy thanksgiving