How to troubleshoot stereo is going into protect mode?

Modern stereos and automotive head units have internal breakers that shut down the amplifier circuits when the output exceeds the rated wattage. They are designed this way to protect the circuits from burning up. If the stereo has a display panel, it may say "protect" or something similar when this happens. Assuming the breakers are working properly, there are at least two reasons for the system to protect itself. One is an inappropriately low impedance arrangement of speakers, and the other is a short, either internal or external. Once the problem has been resolved, the system will automatically restore the amplifier operation. Let's explore each of the possibilities to identify the problem for resolution.
1. Low impedance speaker setup.
In this case, the speakers attached to the output have an impedance lower than the system design specification. When designing or buying a system, you must ensure the impedance of the speakers is at least as high as that specified by the head unit or amplifier manufacturer. For example, if the spec is 4 ohms per channel, your speakers must have at least 4 ohms impedance. If you wanted to connect 2 speakers to such a channel, you could attach two 8 ohm speakers in parallel or two 2 ohm speakers in series. The latter case is called a bridge and is often used with subwoofers to bring up the impedance of the speaker system. If you do not know the impedance of your speakers, use an ohmmeter across the terminals to read it. Typical values are 8, 6, 4, and 2 ohms. Connecting two similar speakers in parallel halves the total impedance, while connecting them in series doubles the total impedance. Examples are shown below.









2. Short: Now, assuming you have an appropriate setup, let's consider a short.
Internal short: A short may be internal to the head unit or amplifier. To determine this, disconnect all speaker wire from the stereo and turn it on with the volume up. If the system goes into protect mode with no speaker wires attached, the short is internal. Due to the huge variety of systems, repair of an internal short is beyond the scope of this tip. If you have an internal short and you're good with electronics, please feel free to open up the unit to look for burned transistors. Otherwise, consult a repair shop or the maker.
External Short: An external short may be in the speakers or the wiring. If one of the positive speaker wires is uninsulated at a point and touches the chassis of the vehicle, the unit will see zero impedance on that channel. Also, if the speaker on the channel has a melted coil, the impedance can be much lower than the design spec for the speaker. The first thing to do is to identify which channel is shorted. Do this by removing each pair of speaker wires and checking the impedance of the wires and speaker together. If it is lower than the spec for the speaker, there is a short. Check all channels in this way. Next, access the speakers for any case that has lower than spec impedance for the speaker. Remove the wires from the speaker and check the impedance of the speaker. If the impedance is lower than spec, replace the speaker. If the impedance is good, check the impedance of the wires. The impedance should be infinite unless there is a short. If the impedance is less than infinite repair or replace the wires.