In MOST cases, replacing the diaphragm in that particular valve will solve the problem. However, there could be other problems ranging from bad wire connections to a bad triac in the clock on that perticular zone, a solenoid gone bad.... even the clock may have gone "bonkers" and dropped a portion of the program for that addresses that particular valve. If you have and can use a volt ohm-meter, check the continuity to the valve as well as the zone output voltage. Resistance should be between 20 and 60 ohms in that circuit, depending on the mfg'r. Try the other 2 valve circuits for comparison. The output voltage voltage should be somewhere around 24-30 Volts AC on the zone terminals.
The Irrigation Specialist Mfg' Az.
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