My chevy ac blows only on fan speed 5 what could it be

I would think either your fan motor is bust. Or it could be a cabling issue.

If you can access the fan motor. Think yours is accessed behind the glove box. Test the fan.

You should be able to get a replacement from a breakers yard.

If not the fan motor is could be a failed switch/speed selector.

Sounds like high blower relay is bad

Sounds like the Blower Motor Resistor. The range from 10 to 75 dollars depending on what kind of vehicle. Pretty easy to change.
Blower Motor Resistor Repair Heater Fan Speed Control Chevy Blazer GMC...

Likely the blower motor resistor. located right next to the blower motor, this device is resposible for reducing the voltage suplied to the motor. when it fails you may loose some or all speeds of the motor

I’m happy to assist further over the phone at https://www.6ya.com/expert/samuel_dda9c48bb7ffbfd3

not on speed 1 or speed 5 would indicate the fan resistor is going bad this is located over near the fan on the right side lower of dash

you will find a blower resistor near the blower motor , it goes into tho the housing with a harness of 5 or 6 wires, replace it

replace blower motor resister

2003 Chevy Impala w/dual zone heat/AC. The fan stopped working in speed 1 adn 2. Do I need to replace the switch?

heater speed switch in passenger side in foot area above toes in heater box one wire plug on it 2- 5-6 mm screws hold it in that is your fan speed switch they are a common issue it is the season for those now usually you loose 1 speed at a time until nothing don't be caught with out that not working at all you windows will fog bad!!!!and no heat period.

i had this same problem in my chevy prism-- my fan regulator had broken. (apparently it usually happens around 100k miles, i hear) in my car, the vents wouldn't even blow air at all on anything but high--- not just AC, but normal vented air as well.

cost about $80 to have the problem fixed by my mechanic in Jersey. hope that helps!