Hi,
Trying to keep this condensed and to the point...
I have a Hobbywing XRotor 60A 4-in-1 ESC. ESC for motor 3 does not get recognized by betaflight or BLHeli 32 but the other 3 are fine. I had a crash in water, completely submerged. Everything is fine and working except for ESC 3.

I get all 5 beep tones at startup and everything is working perfectly with the rest of the ESC's, motors spin up, no errors nothing wrong but motor 3 does not spin up.

What I have done and checked... So far.
No bent or damaged connector pins on ESC, wiring harness or FC.
Continuity between each pin on ESC to relevant pins on FC through harness.
No shorts on ESC that I am aware of.
Tested with multimeter, each mosfet and motor pins and also checked resistance. They are all basically the same, so ESC 3 has more or less the same values as the rest of the ESC's
Motors idle and checked dshot info and status in CLI with 0 errors on the 3 working ones, but number is just an empty line with no info.
All stock configs on FC, updated before incident to latest FC firmware, ESC's were flashed with version 32.7.
Soldered a different motor to the non working esc motor tabs, motor also doesn't spin up and still not recognized in BLHeli.
Replaced wiring harness twice with new ones, no luck
Tried resource mapping to motor 5 pins, checked timers and dma streams for each motor output. Went back to defaults again afterwards.
I see no point in downgrade firmware on both FC and ESC as it worked well for weeks prior to the crash.

All I am left with to still check is the under side of the ESC as the top side has the capacitors and mosfets, relatively easy to check with multimeter. But the under side has a lot of resistors and smaller capacitors etc. The 4 esc components are not directly placed the same order as the other esc components so it makes it difficult to check each component on one esc and verify the value to the same component on the other ESC.
So I am left with trying to figure out how and where and what smd components to check and compare to see if maybe one or two was gone or damaged and gives different values.

I am desperate for help and guidance in anyway possible because I really feel that it is just something small I am missing and that needs to be replaced in order for it to work 100% again. It practically is 3/4 of the way there... Just one ESC to fault find and I can fly again.
I already lost my xrotor F7 convertible flight controller as the 3.3V line feeding the MCU is dead and there is only a small amount of resistance on the 3.3V LDO where it is supposed to have A Lot more resistance according to the videos I have watched on how to fault find components. Everything else works but dead MCU means going nowhere.

Please help me save my ESC!!
Thanks

It's likely that the MCU for that particular ESC has been killed. There isn't much you can do about that unfortunately. Even if you were able to get a replacement MCU and solder it onto the board, BLHeli_32 ESCs are licensed by MCU so a new virgin MCU chip wouldn't have the required (valid) license data on it.

Put it in your spare parts bin and buy a new ESC board.

Some people use a single ESC of a similar spec to replace the 1 broken ESC of the board. Personally I have never done this. I do still have a 4-in-1 ESC with 1 fried ESC on it in a drawer somewhere because I have been meaning to try it out, but haven't bothered yet.

If the board went into water and since then 1 ESC won't work, then unless you want to try using a stand alone ESC to replace the 1 broken ESC then as SnowLeopard said your best bet would simply be to put in a new 4in1 ESC board.

WARNING: Unless you replace it with the exact same model of ESC you need to check the pin-out of the wires connecting the ESC to the flight controller. Different brands and even different models use a different order to the wires. When I first got started in this hobby I fried a brand new never-flown FC because I assumed there was a "standard" of wiring order since they all use the same plugs. Plugged them together without checking the pinout and ZAP : fried brand new FC. Just check the order of the wires and you are fine, or use the same ESC to replace it would be the easiest.

.. Okay, some changes from my previous post.
Been reading and watching a lot of videos for fault finding and managed to isolate my issue to only 2 problems on the ECU.

Motor 2 is not spinning or making any beeps when powering on.
After extensive multimeter probing i found that the motor signal pads going to the flight controller, all show 3.28V except for motor 2 (that is not working). So that is already an indication of something.
I then compared all the multimeter continuity values of every individual esc component to each other and looked for variances where motor 2 components might have different values, possibly indicating a failed component or at least a narrowed solution.
I then checked all the voltage readings on every component on the entire esc and specifically compared the 3.3V positions of each individual esc to each other.

So.. This is what I found and MY interpretation thereof..
Each MCU has 8 pins where I found 3.3v on the pin. According to the schematic it is 2 x Vdd pins (3.3V supply voltage to power the MCU), 2 x pins for SWDIO and SWCLK (as outputs to the exposed pads for programming the MCU), 1 x NRST pin (Reset), 1 x Vdda, 1 x Boot0 pin and finally pin 8 (PA02).
I have the same 3.27V readings on All 4 ECU MCU pins for Vdd, NRST and Vdda, BUT Motor 3 MCU does not have any voltage on the PA02 (#8 pin) and also not outputting voltage to the SWDIO and SWCLK pins/pads.

So this is where I am stuck. I don't know if pin 8 (PA02) is an output voltage or an input voltage supplied. It also has 2 small capacitors connected close to the pin.
If it is input voltage then it my understanding is that there is an issue somewhere on the ESC 3.3V supply rail not reaching that specific location.
If it is output voltage, then it might mean that pin is fried on the MCU and replacing the MCU might fix the problem. The thing is, if it is output voltage, why does it only go to the 2 capacitors and then nowhere from there.

My theory is that, the missing 3.27V on the motor signal pad has to be linked to this pin 8 of the MCU, because that is the only difference between every component on each individual ESC so it has to be.
My problem is not knowing or isolating why there is no voltage on that SPECIFIC pin and capacitors yet the 3.3V VDD supply is present on the same pins as the rest of the other ESC's same pins. So it leaves me to think that there is nothing wrong with the voltage supply rail, but rather maybe a component or either of the 2 capacitors.

I can understand if all this sounds confusing but I will upload a picture of the ESC with markings of all the areas where I have 3.3V present as well as the entire 3.3v rail.
This will show you that the missing 3.27V goes from the pin to the 2 capacitors and then dissappears. I am still unable to trace it anywhere else on the board..
But obviously since this is the only difference compared to the other ESC components, it leaves me to believe that it must be related to my motor not turning on, so somehow it has to influence the Mosfet driver for not driving the motor fet's.

Any advise or guidance to troubleshoot further would be greatly appreciated.. I have a hot air station and a damaged 32bit esc with spares on waiting should I have to replace something, but I don't want to change the MCU and it still is the same result.

Thank you in advance.