Chosen Solution
The backlight turns on for roughly 200-300ms when the computer first powers on but then turns off and never comes back on. The computer is 100% fully working otherwise. It has an image on the display and everything else is tested and works. I have already tried replacing the PSU and inverter board, but there’s no change. PSU output measures normal. Inverter input does not measure normal. The inverter board has 4 pins on the connector: V-in, ground, PWM, and Enable. V-in is the proper 12V, ground is obviously 0v, Enable is 3.3V , and PWM should be 3.3V but I am only measuring 1V. I inspected and measured the appropriate components on the board and they measure according to the schematic. I found a similar question posted by another user with similar board experience who also did the same testing as I have, but a new PSU fixed his problem whereas it did not for mine.
@dpwhitty there seems to be a lot of dancing around on this question. To clarify, you replaced both the power board and the inverter with known-good replacement boards. If so, then the only next option will be to check the panel itself. Common panels for this model was a Phillips (LG) LM201WE3-(TL)(F5) You need to check yours and let us know what your panel number is. VLCD for the common panel should be 5V. This type of panel requires two power inputs. One is to power the LCD electronics and to drive the TFT array and liquid crystal. The second input power for the CCFL, is typically generated by an inverter. Failed CCFL’s often display this behavior “the backlight will come on for that split second before it cuts out. “ but since they are only for the backlight, you should still see the image by shining a flashlight at an angle against the screen. New power board and inverter would then indicate a faulty driver board (part of the panel)
Given the age of the system I would replace the PRAM battery iMac Intel 20" EMC 2133 and 2210 PRAM Battery Replacement as sometimes the settings get lost or corrupted when the battery is low or dead. A failed Inverter could be your issue. Both the power supply and the invert work hand in hand to supply the needed power to drive the CCFL’s for the backlight to work iMac Intel 20" EMC 2133 and 2210 Inverter Replacement and the needed part iMac Intel 20" EMC 2210 & 2133 Backlight Inverter. To verify shine a flashlight into the screen at a sharp angle you should see the faint image of your desktop and its icons. If you do then the issue is within the backlight logic and/or the CCFL lamps. If you’re careful you should be able to measure the voltage going to the lamps (be very careful! This is high voltage).
