link to ebay circuit board.
https://www.ebay.com/itm/315062235001...

Here's how I go about testing these water heater gas control valves. You can do all these tests with the control valve in the heater.

I first test the thermopile for proper voltage. I like to see between 0.4 to 0.5 volts. Low voltage can cause the circuit board to misbehave. This happens because the thermopile is the only source of power the control board has to keep itself alive and move the two selonoids to meter the gas. Poor positioning of the thermopile can cause low voltage. Try bench testing the thermopile with a lighter and multimeter.

Check ohms and power the two selanoids that meter the gas. If the ohms are within specifications on the two posts and they click with power supplied. Coupled with adequate power at the thermopile would make me consider the board is defective and unable to send power to the selanoids. Because the control board is in the middle of the two operations, and each part worked separately.

Constant code 4. I try doing the reset. If the board doesn't reset then the knob positioning could be out of calibration. It's worth replacing the board. You can try setting the water temperature to the lowest setting. If the code 4 goes away than the board could be defective. If the board still has a code 4 on the lowest setting then I'd lean more towards the water temperature sensor being defective. Replace the entire valve at that point.

If the board fails to reset or is difficult to reset then I'd lean to the knob settings are out of calibration.

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