ASUS PRIME B450 PLUS (rev 1.02) USB Overcurrent issue

[ezenia]

Hello,

So I bought a faulty ASUS PRIME B450 PLUS motherboard off eBay. They said it was water damaged and they cleaned it. The PC POSTs and displays a message that it's detected USB overcurrent and will shut off in 15 seconds. I verified this myself. So I did some checks at the ATX connector 1st to see if the resistance readings looked OK. I found the 3.3V rail measures just 179Ohms which seems way too low. It's been much higher on other AM4 boards I've worked on.

So I did some Googling to see what the potential culprits are. It varies from bad protection diodes, bad ports or even the PCH. I went to the protection diodes (they are SOT23-6) and measured the resistances on the legs against GND. All looked OK, but one. One was reading close to the Ohms I was seeing on the 3.3V rail. I attached an image of the 2 readings on the chip:




So I removed it to see if that changed anything. Unfortunately not, the resistance readings remain the same. It seems like only one port is affected. I checked the pins on the USB port at the back of the board, on 2 of the pins for one port, I get the same low ohm reading. On the other port, I don't see the same low ohm reading. The ports are USB 3.1 Gen 2. They're light blue.

Before going further with this repair, I'd like to know if this issue is perhaps being caused by the PCH since the 3.3V rail is also reading low ohms. If it is the PCH, then there's not much I can do about that. If it's not the PCH, then I can continue to find where the issue is.

Appreciate any help on this. It would've been nice if I had a boardview for this board, but couldn't find one (I've requested it in the appropriate section).

[wisjg]

U74LVC1G86G

[Fernon]

U74LVC1G86G

How to check this two ic

[ezenia]

Oh wow, did not see this thread got a reply (only latest reply sent me an email notification). Guess I'll provide an update on this issue.

I somehow managed to get this board working a while ago. Not really sure what I did to fix it. After trying a bunch of stuff (like removing the 6-pin IC), I decided to try some voltage injection (used 1v at a very low current limit) into those pads reading 173/137 ohms (put one probe on ground). After doing this, the resistance went high and the OC message went away. Affected port was now dead though. Did some more testing and found if measuring the resistance between data+ and data- of the affected port, the reading was just 46 ohms which seemed low. Checked other working ports which all showed very high resistance between their data pads. Found a boardview to see where the data pads go and found it only goes to that 6-pin chip and then directly to the PCH. Chip was not the issue as with it removed the reading was still 46 ohms between data+ and data-. Port was not the issue either as I did try desoldering it. That left me thinking it was a PCH issue (partial short from the looks of it).

Decided to see what happened if I injected some voltage into data- and data+. Not much happened, only a flicker of current and then nothing. Re-measured resistance and it started reading high resistance (same as other working ports). Booted and now the top port works and still no OC prompt at boot.

Since the "fix", I've regularly tried power cycling the board to see if the issue returns. So far it has not. Very strange issue. EBay listing did say the board was water damaged and then cleaned. Maybe the injection cleared some minor corrosion that I couldn't see or something. Who knows. All I know now is the board is so far working OK.