Okay, let's break down the likely scenario and how to troubleshoot this. It sounds like you have a fan/light combination or a fan with a separate nightlight, and the red wire is likely for controlling one of those functions.

Understanding the Wires



White

Neutral. This should always be connected to the white wire from your house wiring.

Green

Ground. This should always be connected to the green or bare copper wire from your house wiring.

Black (one with yellow tape)

This is almost certainly your

main hot wire, likely the same hot wire as is connected to other lights/outlets in the bathroom. Black (plain)

This is likely the hot wire going to the fan itself.

Red

This is likely the hot wire going to a secondary function, such as a light or a separate speed setting on the fan.

Troubleshooting Steps


Safety First! Turn OFF the circuit breaker to the bathroom before doing any further wiring. Double-check with a non-contact voltage tester to be absolutely sure the wires are dead.2. Verify Your Connections

Disconnect all the wires you connected earlier. Then, using the non-contact voltage tester, turn on the breaker and identify which of the two black wires is the hot wire. Turn the breaker off and continue below:3.

Connect the Essentials



White to White. Securely connect the white wires together. Green to Green. Securely connect the green wires together. Black (with yellow tape) to Black (from the house wiring). Securely connect the main hot wire to the fan's main hot wire.4. Test the Fan

Turn the breaker back on. Does the fan operate?

If the fan works

Great! You've identified the main fan circuit. Now you need to figure out the red wire. Proceed to step 5.

If the fan still doesn't work

Double-check all your connections from steps 3 and 4. Make sure the wire nuts are tight and that the wires are making good contact. If it still doesn't work, there might be a problem with the fan motor itself, or the hot wire you chose is not the right one.
5.

Identify and Connect the Red Wire



Do you have a second switch in the bathroom that controls something other than the fan? If so, the red wire is almost certainly for that function. If you want the fan and light (or whatever the red wire controls) to operate with a single switch

Connect the red wire to the same black wire (with yellow tape) from the fan that you connected to the black wire from the house wiring. This means all three wires are connected. Keep in mind that you will not be able to control the light independently from the fan if you do this.

If you want to control the fan and the light (or other function) separately

You'll need to connect the red wire to a separate switch leg. This is not a DIY project and requires specialized electrical knowledge. Call an electrician.
Important Considerations



Wire Nuts

Use appropriately sized wire nuts to make secure connections. Make sure the wires are twisted together before you screw on the wire nut.

Local Codes

Electrical codes vary. If you're unsure about anything, consult a qualified electrician. This is especially important if you're making changes to the wiring beyond simply replacing a fixture.

If you are unsure at any point, call a qualified electrician.* Test

After everything is connected, test the fan and any other function connected to the red wire to make sure they are working properly.

Disclaimer: I am an AI chatbot and cannot provide definitive electrical advice. This information is for general guidance only. Working with electricity can be dangerous. If you are not comfortable or familiar with electrical wiring, please consult a qualified electrician. I'm not responsible for any damage or injury that may occur as a result of following these instructions. Flag for review