There’s an issue that plagues technicians and consumers alike. When a Microsoft Surface Pro 4 suddenly no longer powers on. Sometimes this happens seemingly randomly, other times, immediately following a repair.

Parts of this puzzle have already been answered elsewhere on the internet. But what happens when its “neither of those”?

With no available schematics, it’s difficult to determine the fault. We had a few of these in the queue, so we decided to reverse engineer one. We’ve put together a list of the 3 most common failures to check for, in order of likelihood.

1. A bad Schottky Diode If you made it here from a search engine, you’ve probably already seen this failure 10 times over. A diode is effectively a one-way valve. It allows power to travel in one direction, and blocks it from traveling in the other. Unfortunately, a diode also causes a drop in voltage as power passes through it (known as the “forward voltage”). This is generally not ideal or efficient, and the voltage that is lost results in heat dissipation. This effect worsens the more current the diode passes. Diodes also have a rated maximum current. Exceeding that current (say by overloading or short circuiting) can result in failure. Diodes fail in one of two ways. Either the forward voltage increases substantially (sometimes infinitely), or the diode effectively turns into a wire, allowing current to flow in both directions. A Schottky Diode is a slightly “better” diode. These are commonly found in all kinds of electronics. Schottky diodes have a lower forward voltage compared to their silicon counterparts, are available with higher current ratings, are more efficient, and as a result allow for higher switching speeds. The Surface Pro 4 has an abundance of Schottky Diodes on the motherboard. In reality, any of them have the potential to fail. In most cases though, one of the diodes pictured here will fail. These diodes allow either the battery or the charger to power the device. The use of diodes means that whichever input is higher voltage will feed power to the unit, without feeding power backwards into the other input. For example, with the charger connected, the charger will power the unit, without feeding the battery and potentially over-charging it. Conversely if you leave the charger connected but not powered on, the charger won’t cause the battery to drain. If you have a lower end Surface Pro 4, you may find that the “charger” diode is not populated. This is because the dedicated charging IC has enough capacity to charge the battery, while the battery simultaneously feeds power into the motherboard. In either case, a failure of the diodes will present with the following symptoms: Battery: A failure of this diode will result in the device immediately turning off when the charger is disconnected. This won’t effect charging the battery, or the battery reporting its state to the operating system, but will prevent the device running from the battery. In the case of a lower end unit (without the “charger” diode), the device will not function at all.

A failure of this diode will result in the device immediately turning off when the charger is disconnected. This won’t effect charging the battery, or the battery reporting its state to the operating system, but will prevent the device running from the battery. In the case of a lower end unit (without the “charger” diode), the device will not function at all. Charger: A failure of this diode (on higher end devices that have it) will result in the unit running from the battery, but not the charger. This may mean the device says it is charging, but the percentage decreases as it gets used. Alternatively (depending on the mode of failure), the device may not charge at all.

A failure of this diode (on higher end devices that have it) will result in the unit running from the battery, but not the charger. This may mean the device says it is charging, but the percentage decreases as it gets used. Alternatively (depending on the mode of failure), the device may not charge at all. Spare: Funnily enough, on all of the devices we have seen so far, Microsoft was kind enough to provide us with a spare Schottky Diode. On the boards we checked, both sides of this diode connect to ground, and as such, it does nothing at all. We aren’t quite sure what happened here. Either there are variations of the SP4 that have this diode in use, or the part was accidentally left in the pick and place queue and nobody noticed. In either case, it’s handy to know in the event of a single diode failure, there’s probably a spare waiting for you in the device. Testing the Diodes So if a diode is bad, how can we tell? With the battery isolated (this is important) use a multimeter in forward voltage test mode (diode mode).

Measure across the diode leads. You should have no more than 0.250V forward voltage, and no less than 0.100V.

With the probes switched around, you should get a very high voltage (like 2.800V), or “OL“. If you get close to 0.000 in both directions, your diode is shorted and needs to be replaced.

If you get a high reading or “OL” in both directions, your diode is blown and needs to be replaced. A burn or hole is also a pretty good indication. To replace the defective diode(s), you may be able to use the “spare” that we have noted above. If you don’t wish to do so, or if you have more than one defective diode, we also sell replacements in our store.



2. A bad Fuse If the Schottky Diodes are good, or you replaced them both and the device does not work as expected, the next thing to check is the “fuses”. There are actually a number of these on the board. So many that we can’t list them all. We’ve circled the most common “fuse” to cause no power. There are 3 variations of these fuses we have seen. In most cases they are a 0 ohm resistor, either marked “0”, or “000”. We still call them fuses, but in reality, they are 0 ohm resistors, aka “links”, aka “jumpers”. Less commonly, Microsoft chose to populate these with actual fuses, typically marked P or L. If you locate the fuse we have circled, whatever it looks like is likely going to be the same for all the others. Testing the Fuses If you’ve ever used continuity mode on your multimeter, you will be quick to grab it out and check for the “beep”, indicating a connection.

Before you do though, be warned that this method is NOT fool proof, and may lead to misdiagnosis. While a lack of continuity would certainly indicate a “blown” fuse, a 0 ohm measurement or “beep” certainly does not indicate a working one. This actually applies for all kinds of devices, and we learned this the hard way on iPhone Backlight fuses many years ago. To correctly test the fuse, you need to pass some current through it. Your multimeter uses a very small amount of current to test for continuity, and it isn’t enough. There is a common failure mode where the fuse will appear to have continuity, until you try to pass current through it. To confirm operation of the fuse, we set our DCPS to 1V, and 0.05A (50mA) to 0.10A (100mA).

The voltage here really doesn’t matter, but its a good habit to use less than the circuit is designed for. This prevents damage to components should you accidentally slip and power the circuit. With probes connected to the DCPS, we verify the current limit by touching the probes together. The “CC” (Constant Current) LED on your DCPS should illuminate.

Now, place your probes either side of the fuse. The DCPS “CC” LED should illuminate, and the voltage should drop down to 0V. If it does, you have confirmed your fuse can pass current.

If it does, you have confirmed your fuse can pass current. If the “CC” LED does not illuminate, if it doesn’t stay illuminated, or if your voltage does not drop to 0, your fuse is not working properly. To replace the fuse, you can use a 0 ohm resistor, an iPhone/iPad backlight filter/fuse, or even a piece of wire (although we would advise against it).