YOU CAN NOT repair combination gas valves. It is not "code" there are no parts available, and there is no repair information. PERIOD.
Found this
Honeywell VS820A1088 Millivolt Combination Gas Valve
OP what is wrong with the heater/ valve? That is what is known as a "millivolt" or "self powered system
SOME STUFF!!!
HOW THEY WORK A thermopile gas valve does several functions all in one big box
1...There is a pilot tap-off and on most newer valves, the pilot is shut off "safe" in the even of a flame failure which allows the thermopile to cool. In any evet cooling of the thermopile due to loss of flame requires 100% shut off of AT LEAST the main gas supply.......with NO leak
2...The pilot may or may not be a modern "oxygen depletion setup." Regardless
the pilot MUST be kept clean or it will not generate enough heat to reliably keep the thermopile engaged. Remove pilot assy and tubing pull it all apart and blow it out with air. "Look" through the pilot orifice and visually confirm it is open before and after you blow it clean. Do not try and clean it with wire--it is too small
3....The thermopile really does 2 jobs---it's heat and generated power holds the safety open so that the valve can operate. In the event of loss of flame the cooling and loss of power at the thermopile causes the valve to close. In addition, the power output opens the main burner valve during normal operation..
4....Thermostat voltage drop (dirty contacts and type of stat, etc) as well as SIZE AND LENGTH of stat wire cause these to be "fussy" "on the edge" or undependable. ALWAYS bypass the stat (if used) right at the valve to see if this is a problem
Also, some appliances have additional series wired "safeties" in the millivolt chain. ANY thermo-switch/ safety in series adds the probability of voltage drop in these "on the edge" systems. Follow the circuit diagram and find out.
5....Thermopile output regardless of internet wisdom is difficult to test. If you have bypassed the control wiring to test, cleaned the pilot, and the thing still will not operate, always replace the thermopile, and make certain it is properly IN the pilot flame
Thermopiles FAIL and are always "on the ragged edge."
You do NOT absolutely have to replace that valve with the same part number. So long as the valve can handle the BTU range of the heater (IE it is not too small for the gas flow), so long as it is a millivolt/ thermopile type valve, and will physically fit in the space in the burner, you can sub other brand valves.
ALSO. If there is no other appliances on the piping to that unit CONFIRM that you have adequate gas pressure TO the appliance. This requires a low pressure measurement such as a U-tube manometer or the electronic equivalent. Pressures are different for nat vs LP gas EG it might be that somehow water has gotten into the system. THIS WOULD BE RARE but has happened. Water might be "in a low spot" in the piping. Gas piping occasionally DOES become plugged, sometimes with foreign material that had ALWAYS been in the pipe, and finally just migrated enough to restrict flow. This problem however is usually indicated by a good pilot....good ignition of main burner........followed by "petering out" of the flame as flow drops off
I spent about a dozen years in the 80's --early 90's servicing HVAC / R
