How To Test A Car Stereo – Steps, Diagrams, And More!

As an installer, I saw a lot of used car stereos that customers brought in for installation but ended up being bad. In fact, it happened so often we never would install a car stereo without bench testing it first.

It’s so important to know how to test a car stereo before investing your time & money installing it. That’s why I put together this article – to help you do it yourself!

Here you’ll learn:

  • How to connect & test a car stereo at home & without a car (no, you don’t have to install it!)
  • What you need to power & test a car stereo
  • How to use a computer power supply or car battery as a power source for testing
  • Helpful tips and diagrams to make it easier
  • Common problems with bad car radios

Basics first: what you need to know before testing a car stereo

Image of teacher instructing how to test a car stereo

The truth is that you don’t really need to “stress test” car stereos. Especially with second-hand and used radios, the real goal is just to be 100% sure your car stereo works before you go through the trouble, time, and effort of installing it.

Fortunately, it’s actually fairly easy to do. You’re going to need to do just a few things:

  • Connect it to a power source properly so it can turn on.
  • Test the speaker outputs: I’ve seen quite a few car stereos where the former owner burned out the speaker outputs due to a poor or incorrect installation.
  • Check the main functions: CD player (if present), radio, Bluetooth, and volume controls.
  • Check for worn-out buttons and other parts.

The great news is that you don’t need to install a car stereo just to check it properly. In fact, you can use just a few simple things and be done in a little while. You’ll need the following:

  1. A test speaker you know is good [see below for suggestions]
  2. +12V to 13.8V power supply (or a vehicle battery – more about that later)

In this article, I’ll cover the steps for testing a car radio, how and which power supplies to use, and the important details to help you before you start. If it’s your first time, I’d plan for spending about 10 to 30 minutes of your free time going through the steps.

Once you’ve done it, however, it’s a lot faster the next time.

Standard car stereo wiring colors: wiring color chart & notes

example of a car stereo wiring harness with standard wire colors

Most – but not all – car radios have a wiring harness that’s color-coded to make installation easier. Power and speaker wiring is usually color-coded while some additional specialty wiring depends on the brand. Note that not all manufacturers follow the wiring color standards so it’s best to check to be sure.

Standard car stereo wiring color chart

Above is a handy chart to help you connect the speaker and power wiring you’ll need when testing a car stereo. Car stereo harnesses you find on most – but not all – brands use standard wiring colors that tell you exactly what they’re for including power, ground, and the speaker connections including positive & negative markings.

This is especially helpful if the radio’s wiring label is torn off or damaged like happens with older radios or if it’s nowhere to be found. Note that some specialty wiring like the dimmer input wire, a mute input for a GPS system, or a few others may use custom colors that only that car radio brand uses. 

Not to worry though as those you won’t normally need to bother with just to test and check out a radio.

Note: Standard car stereo wiring colors are used by many but not all manufacturers. It’s always best to double-check to avoid any problems.

What you’ll need: speaker & power supply notes

Examples of items needed to test a car stereo at home

The good news is that it’s actually pretty simple to get what you need together for testing a car radio. What’s great is that you’ll only need a few parts to get the job done:

  • A test speaker: This can be nearly any full-range or midrange 4 ohm or higher speaker (yes, home stereo 6, 8, and 12Ω peakers will work). Using a tweeter can work but I don’t recommend it as that’ll make it harder to hear sound from the radio’s speaker outputs. You want at least a midrange or full-range speaker.
  • A 12V or 13.8V DC power supply: you’ll need about 1 amp (1A) or more of current for powering on, getting sound from, and checking out most car stereos. You can also use a vehicle battery or computer power supply if you don’t have a 12V DC supply handy.   See the detailed diagram and section below to learn more  
  • A test CD for CD players and/or Bluetooth device: For modern car radios with Bluetooth functions, a smartphone is usually fine to get a Bluetooth audio test source. For CD players it’s best to try out the CD function so you’ll want a disc that you know is good.
  • Electrical tape or similar: not a requirement, but it’s much better to insulate speaker wires you’re not using so they can’t short to the radio’s metal body or each other. Duct tape or other kinds of tape can also work.
  • Needle-nose pliers, cutting pliers, or wire strippers: you’ll need to strip insulation from the radio wiring for testing so a decent tool you can use to remove the wiring insulation is needed here.

Note that while most car stereos use 4Ω speakers, there’s no harm in using a higher impedance (higher Ohm) rating speaker for test purposes.  Any speaker that’s 4Ω or higher will work.

You wouldn’t want to higher impedance speakers every day in your vehicle for the reasons I explain here, but when just checking the speaker outputs anything handy will do.

The main thing is to use a speaker you know for 100% sure is working. Don’t guess – be 100% sure your test speaker is good and plays well when connected to a radio or you could mistakenly think your car stereo has a problem when it doesn’t when testing it.

Note that 1 amp is usually enough to power a car head unit just enough for checking the functionality and getting sound with a test speaker. For driving a car radio with higher power output to multiple speakers, you’ll need more current (a higher amp power supply).

How to test a car radio (diagram and notes)

How to test a car stereo diagram

How to test a car radio

To test a car radio, there are 3 main things you’ll need to do:

  1. Connect the power and ground wiring
  2. Use a test speaker on the speaker outputs to verify they work
  3. Try out any other audio functions you’ll need to use

1. Connecting power to the radio

As I mentioned earlier, you can use a +12V to 13.8V DC power supply for your car radio. Optionally, if you don’t have a 12V supply you can temporarily connect it to a 12V car battery long enough to check out the radio.

In either case, you’ll need to do the following:

  1. Connect the +12V BATT and ACC wires together then to the +12V source. You’ll need both wires to be connected for the radio to turn on. Connect both of these to the +12V output of your supply or the battery.
  2. Connect the ground (“GND”) wire to the negative supply or battery terminal.
The red wire, “ACC” is a +12V input that signals the car stereo to switch on with the ignition, while the yellow “BATT” wire is the main power source. The ACC wire must have power for the radio to work.

Be sure to insulate any unused exposed wires including the speaker wires (more about those later). You don’t want any wires shorting together during testing. For insulating them, electrical tape works great or you can use duct tape or even masking tape carefully if you must. I’d avoid transparent Scotch tape as it tends to come off too easily.

The idea for testing is to temporarily do what you need to for checking that it works. For a real vehicle installation, better connections like crimp connectors should be used.

CAUTION: If using a vehicle battery to power you radio under test, never allow the metal case of the radio near the battery. The metal body is grounded and could cause a dangerous short ciruit if it were to touch a positive wire or terminal.

2. Testing the speaker outputs

Example of the rear of a car stereo installation closeup

Most car stereos use an internal amplifier integrated circuit (IC) chip to provide the power to drive speakers in a vehicle via the speaker wire outputs. Bad speaker outputs due to improper installation (short circuits, low speaker impedance, etc) are a common problem with used car radios.

It’s important to check these and make sure they’re working as expected before bothering to install it. To do so, you can use a test speaker as mentioned earlier with an impedance of 4Ω or above. Even a 6, 8, or even 12Ω speaker you’ve got at home works great for this!

To test the speaker outputs:

  • Connect a test speaker to each speaker wire output pair one at a time, being careful to keep the wires from touching. Keep unused wires insulated with tape or connectors to prevent a short.
  • Turn on the car radio and play music from the FM radio tuner or other functions.
  • A properly working car stereo should have good speaker volume and sound output.
  • Repeat this for all of the remaining speaker outputs.

Note that of all the car radios I’ve tested, it’s not unusual to run across used radios that have no audio from the speaker wiring. 9 times out of 10 this is due to a poor-quality installation where the wires were allowed to touch, someone trying to “get more power” by wiring it the wrong way, or using the wrong impedance (Ohms) speakers.

When a car stereo’s amplifier chip is damaged, it’s permanent and the output transistors fail, no longer able to produce a musical signal. There’s no solution for this outside of paying for a manufacturer repair or just replacing it.

Note: The good news is that generally you shouldn’t need to test RCA jack, if present, on your car radio. These rarely if at all fail. Speaker wiring outputs are the main concern.

3. Testing other functions

Example of testing a car stereo Bluetooth audio function Kenwood KMM-BT518HD

Be sure to test the other functions you’ll use the car radio for: USB audio, CDs, HD radio, Bluetooth, phone direct control (USB cable connection), or others.

Finally, if the speaker outputs appear to be good to go, you’ll want to be sure to check the other main audio functions you’ll be using regularly:

  • The CD player: should be able to load a disc, read the tracks, play it and change tracks, and eject it with no problems.
  • HD radio: if available, be sure it’s working ok. You’ll need to connect an FM radio antenna to try this, however.
  • Bluetooth audio/streaming: you’ll need to connect your smartphone and try this out, making sure it connects normally to a Bluetooth device. (NOTE: If too many devices were already added, you may have to delete one or more from the radio’s memory to free up a new connection)
  • USB/memory card playback: You’ll want to put some .MP3 files on a USB flash drive and/or an SD or microSD card as needed to be sure it can read and playback from them. If a media device fails, there’s likely either a memory card/flash drive formatting problem or the radio itself has a problem.

Also, always be sure to try out most of the buttons to make sure they work properly and aren’t worn out. 

How to know if a car stereo’s controls are in good condition

Rotary volume controls should have a good feel to them and the volume should adjust up or down without a problem. Buttons also should have a good “feel” and shouldn’t be stuck, fail to respond (a sign of a worn-out button switch), or do nothing.

If one or more buttons don’t work or seem right, that’s a red flag that the car radio has either been abused and may not last too much longer. Faceplates can be replaced for brand name head units, but you can expect to spend $50 or more easily for a replacement if you can find one!

One big sign of a car head unit with a ton of use (and wear) is that the printed labels are worn off or show signs of wear already.

How do you hook up a power supply to a car radio?

How to wire a computer power supply to a car stereo diagram

As I mentioned in the beginning, you can use a PC computer “ATX” power supply you have handy to power a car stereo for testing. It’s not that hard as you only need a few steps:

  • Power connections: Cut a +12V wire (yellow) and a ground (black) wire from the main connector. Strip the insulation to leave about 3/8″ to 1/2″ bare wire. Use a crimp connector, solder, or another connector type to join the power supply’s +12V output (yellow) to the radio’s +12V battery wire (yellow). Do the same for the ground wires (black).
  • Supply on control: PC supplies don’t turn on even if the on/off switch on the case is used. A PC motherboard uses a control signal to the “supply on” wire pin. To do the same, you’ll need to find, cut, and jumper this control signal wire to a ground wire either directly or with an on/off switch if you like. ★ See diagram above ★
  • Radio accessory wire: Connect the radio’s accessory/on wire (red) to a +12V power wire from the supply either directly or you can use an on/off switch if you like.

Of course, you can use only one +12V wire from the supply and connect it to both the radio’s ACC and battery wires together if you like. Once power is connected to the radio, connect the “supply on” wire in the diagram to another ground wire as shown. The supply should start and your car radio should turn on. 

Note that some car stereos are switched off by default when they’re first connected to power so you may need to check the power button.

NOTE: Switching off the power supply will cause the radio to lose its “memory” (settings, last station you played, etc) so you may find it easier to use the radio’s power on/off button until you’re finished testing it.

What if my car stereo isn’t working right?

Man thinking about bad car stereo failed testing

I’ve seen some odd problems with used car stereos over the years. It tends to be the same group of problems over and over again – sadly, in a lot of cases, it’s not economical or practical to deal with the headaches.

Here’s a list of problem you might come across and what your options are:

  • No sound from the speaker wiring: If you’re sure you’ve got a good test speaker and there’s nothing else going on, unfortunately, if you don’t have sound the radio’s outputs are likely burned out or have failed. For a few rare models, the internal speaker amplifier can be turned off so you’ll want to be sure that option isn’t causing the problem. Otherwise, it’s most likely the radio is bad and you’ll need to replace it.
  • Won’t turn on with power correctly wired: If you’re 100% sure the wiring is correct, I recommend double-checking for voltage at the pins in the radio’s plug-in harness connector with it unplugged. Some units have an inline fuse you’ll want to check and replace if needed. If the fuse blows again after replacement and the radio still won’t turn on, it has an internal problem and it’s a dud.
  • Controls or volume only partially work/bad buttons/etc: For radios with control buttons like presets, up & down volume, or others that do nothing when pressed, it’s probably because they’re broken or just completely worn out. A clue to this is that there’s no tiny “click” feel when you use them. Bad buttons have no “feel” to them – they’re sort of “stuck” and don’t move. You can try finding a replacement faceplate to remedy this; otherwise, it’s time to find another radio.
  • CD won’t load/can’t read a disc/CD is stuck inside: CD players that can’t read a disc often have a laser lens problem and need repair or adjustment. If a CD can’t be loaded, it’s often a mechanical failure and will require replacement of the whole CD assembly (not easy to find). CDs that are stuck can sometimes be removed safely, but you’ll have to take apart the whole radio, carefully remove the CD, re-assemble it, and try it again. More than likely, however, you’re better off replacing the radio.
  • The faceplate is “loose” and/or it has to be held in place to work: some detachable-face radios can develop a poor connection where the faceplate connects to the main body. You can try some contact cleaner for the faceplate’s electrical contacts, but most likely it’s a bigger problem.

There are always exceptions, but these tend to be some of the most common problems you’ll find when testing car stereos. Thankfully, more and more sold these days use fewer mechanical parts so they’re less likely to have the same problem (aside from wear and tear of the buttons).

If after testing you discover your car stereo isn’t working right and has some issues (especially if there’s no sound from the speaker outputs) my advice is to move on and get a nice condition replacement with modern features.

For under $100 these days you can get excellent sound, features, and customization that didn’t exist a few years ago. It’s not worth the headache of trying to deal with a broken car stereo in most cases (unless it’s vintage and rare items).


About the author

Marty is an experienced electrical, electronics, and embedded firmware design engineer passionate about audio and DIY. He worked professionally as an MECP-certified mobile installer for years before moving into the engineering field. Read more »

Your comments are welcome.
  1. THANK YOU! your simplified version answered things I couldn’t find! I recently had an accident total my car I had precious cds in changer. but not precious enough to pay what they want for a second of power to eject from factory stereo. found the acc leads online and now know I don’t need a fuse to hook straight to a battery. Odd enough THAT was what I couldn’t find anywhere else,,not looking to make a “DYI test bench”. You’re awesome Marty!

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