My car skips low fan speed?

Hi, i have a 2007 ford mustang gt manual. The cooling fan seems to skip the low speed mode. The high speed does work normally, it turns on and off normally. Also, when i turn the ac on, the low speed fan does not come on, only the high speed fan comes on. It does not cause me any issues, but the fan does make a high nose and is very noticeable when idling. I noted that the cooling fan that my car has is not the original, it has an aftermarket fan. I'm thinking that it could be the fan resistor that went bad. I already tested the relays and they work fine. Any advice?

Very common problem, it's the blower motor resistor mounted on the side of the blower fan housing under the dash. It will have 4 large gauge wires connected to it.
Make sure you clean out any leaves that may have restricted airflow and install a new cabin filter if it has one. Poor air flow is what kills them.

Faulty contacts in the fan switch.

The low speed engine cooling fan should come on around 214F engine coolant temps… High speed comes on around 225F or with AC on. It could be a bad resistor for the low speed circuit. Test to see if you get power in at the resistor from the relay and power out to the motor for the low speed circuit (assuming engine temps above 214F or disconnect temp sensor) … Should be lower voltage than 12V since the resistor lowers voltage to run the fan at a lower speed. In high speed mode, the resistor circuit /relay bypasses the resistor to send full power .12V… To the fan.

If no power in at resistor, then check the low speed relay/circuit for defects or possibly a bad temp sensor. Some boards suggest the PCM may need to be recalibrated/reset to get proper signals from sensors. If coolant temp sensor reads lower than actual engine temps, then it won't kick on the cooling fan properly.

The low speed engine cooling fan should come on around 214F engine coolant temps… High speed comes on around 225F or with AC on. It could be a bad resistor for the low speed circuit. Test to see if you get power in at the resistor from the relay and power out to the motor for the low speed circuit (assuming engine temps above 214F or disconnect temp sensor) … Should be lower voltage than 12V since the resistor lowers voltage to run the fan at a lower speed. In high speed mode, the resistor circuit /relay bypasses the resistor to send full power .12V… To the fan.

If no power in at resistor, then check the low speed relay/circuit for defects or possibly a bad temp sensor. Some boards suggest the PCM may need to be recalibrated/reset to get proper signals from sensors. If coolant temp sensor reads lower than actual engine temps, then it won't kick on the cooling fan properly.