I currently have the exact same model and so far as I can tell the exact same issue.
My research thus far seems to indicate that some of the LED driver components on the power/main combo board are most likely bad, specifically the voltage regulators (of which there are quite a few in the LED driver portion of the board). I found a random post on an old TV repair forum where someone chimed in stating that the voltage regulators are notoriously bad on all Vizio TVs manufactured around this period of time, they seem to fail most often when the TV is moved. The same poster then later stated that people selling spares on Ebay RARELY, if ever, test the boards beyond the power indicator light coming on and behaving as expected.
So this makes me believe that your replacement board could also be suspect, and should not be considered 100% good, known working.
I am eventually going to try testing and troubleshooting the LED driver components on mine. Currently trying to run down whatever diagnostic info I can (and look up data sheets on the VRs that are present on the board), hard to test if you don't know what you should be testing for.
Detail of my TVs problem and my logic (please question my logic if you see any potential problems).
1. Power indicator light behaves normally.
2. Backlight comes on during initial boot (vizio logo) for ~1sec (lights evenly and, as best I can tell, a normal brightness)
3. Backlight goes off after ~1sec
4. Backlight comes back on for ~1sec when TV displays initial input setting/info (again evenly and at reasonable brightness)
5. Backlight goes off again after ~1sec
6. Screen displays "no input", "no channels programmed", or good input signal properly (but obviously without backlight)
1. LED backlights do briefly come on and appear normal, thus I believe they are OK.
2. Video inputs, panel display, power indicator and remote functions operate normally, so IR/etc. board, power button board, TCON board and functions on the power/main board, other than the LED driver, all function normally. So that really only leaves the LED driver portion of the power/main board as the culprit.
I have read accounts of the LEDs themselves failing which results in portions of the screen being dark, or the screen never lighting up at all.
And by all accounts you replacing the power/main board with a known good one should have fixed it. But given that this is a very common problem with all stock E40-C2 boards, and parts sellers are not 100% about testing full functionality of their parts...
I will try and run down enough diagnostic info to test the state of my board in the next week or so, will relay that info here once I have it if you want to test your board(s). And will report back with any further successes or failures as they occur.