Normally that's the mono valve. If it's voltage goes to zero it will open fully and produce full heat. It's controlled with a PWM signal that controls the amount of heat it allows.

Check the voltage at the connector. On mine the connector got sloppy and tightening the connector contact fixed it.

It could also be a torn diaphragm though this happens slowly over time not generally all at once.

You can bypass it or put a constant 12v on the connector as an emergency fix.

Good luck

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