Hello, the name's Jordan
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Sounds like a power issue. If you don't have power on both the constant and the switched ignition wires for the head unit you'll usually end up getting the little LED light on the unit so it looks like it should be working ok but the screen won't turn on and the unit won't work. If you can take the head unit out yourself and access the wires behind it then here's a simple walk thru that you can use to pin point the issue.
First things first, before taking the time to remove the head unit, check your fuses in the car fuse block. If your fuse box cover doesn't give a description for each fuse, then look in the owner's manual for the car. (flip to the very back index under "F" and it will have a page number for "fuses". The first one to check will be the Radio fuse. If this one is ok, then also look at the following:
Accessory, cig lighter, instrument cluster, interior lights/domelights, audio, and amplifier. Not all cars have the same fuses and names, so just look thru the descriptions carefully and use your best judgement. In some vehicles the radio actually shares a power circuit with the things I just listed above, even though you wouldn't think it makes much sense. Also, keep in mind that most cars have multiple fuse box locations so if you don't see it on the interior fuse box, be sure to check the underhood fuse box too. Some cars will have a fuse box under the driver's side dash, another on the passenger side, another under the hood, and some cars even have them in the trunk or under the back seat.
If all these fuses check out ok then you'll need to dig a little deeper. Here's where you'll need to pull the head unit out of the dash and access the wiring. If you unplug the main radio harness from the back of the Pioneer you'll notice that there's a fuse here as well. They're usually a red, 10 amp mini ATC fuse. If this fuse is ok then continue on.
In order for a radio to turn on and work you need to have 3 wires connected properly. With just these 3 the unit will turn on and act just the way it should but you wouldn't hear any sound without the speaker wires being connected etc.
The 3 main wires are Power (aka 12+ or battery), Switched Ignition, and Ground. The Power wire is YELLOW and is considered a constant power wire. This feeds just enough power to the radio at all times to retain audio settings, radio channel favorites/presets, the clock and so on without killing your car battery.
Switched ignition is the RED and this wire only provides power when the key is turned on. Technically you can wire both the red and yellow wire to ignition only or constant/battery only but then every time you turned the car off you'd have to set all your favorites again, redo your audio settings and reset your clock OR if you wired them both up to constant instead your radio would stay on even when you turn the key off and you'd come out to dead battery every morning.
The ground wire is BLACK. This should be either connected to a ground wire in the radio harness, or if the specific car side radio harness doesn't have a ground wire, you could also connect this wire straight to metal inside the car.
If all the fuses are still good in the car using a voltmeter or multi-meter with a voltage setting and put the black terminal lead on the BLACK ground wire and the other terminal lead (usually red) on the YELLOW constant wire first. With the key turned off you should see anywhere from about 11 to about 13 volts. If you do, that's good. If not, you have a fuse blown somewhere in the car or a bad connection on this wire between the car harness and your radio harness.
Now, keeping the black lead on the ground wire, put the other lead on the RED Switched Ignition wire. With the key turned off thise should read at 0 volts, now turn the key on and you should get again somewhere between 11 and 13 volts. If you don't have power in this wire with the key turned on then again you have a blown fuse somewhere that you missed or a bad connection between the car and radio.
IF you didn't get any power on EITHER wire with either the key on OR off then you probably have a
bad ground connection. Put the black lead from your meter onto bare metal and repeat the last steps with the YELLOW and RED wires. If you get voltage now like you should have in the above steps then you know you have a bad ground connection between the car and the radio. Double check the ground connection or just attach the black wire from the Pioneer directly to bare metal in the car. Good luck
