Hayward H250 (millivolt) pilot won't stay lit

First locate where to light pilot. You can do this by following thin aluminum colored tubing from control valve to inside heater.In general on top of your gas control valve is a knob. Turn indicator of this knob to pilot and depress knob. This will start gas flow to pilot at end of thin tube. Hold this knob in the depressed position and light pilot. DO NOT release knob for 30 -45 seconds. Then release knob and pilot will remain lit. Then turn knob to on position. If pilot will not stay lit you may need a new thermo coupler, which senses heat from flame to keep pilot gas flow open.
Good luck

cannot bypass need new one

you prolly have an electrical short somewhere. follow the circuit and if any safety is getting around 200-250 dc millivolts then theres the problem. otherwise you have an open safety switch, or tsta controls

The lighting instructions are usually located inside the heater on the inside of the access panel or on the firewall near the gas valve.

Most millivolt heaters require you to turn the gas valve knob to the pilot light position and then depress the knob while lighting the pilot through the pilot hole. This varies depending on the exact model.

This is definately hands down a bad thermocouple. The thermocouple sits at the bottom of the burner tray. You will need to remove the nut secureing the pilot assembley and remove the wires.. I will post a link to the manual for this heater below so you can look at the parts diagram to find the thermocouple placement if neccessary. It will also have the part number so that you can order it.

Note: thermocouples can be generic and purchase at a local HVAC or pool/spa retailer.

Refer to page 28 for a diagram of what the pilot assembley looks like. The thermocouple is here in figure 46. Written Instructions for removal of the pilot assembley is on the bottom right of page 28 as well.

Unit dosn't seem to bve sensing flame. Clean your flame sensor. Its connected to your pilot light assembly usually. Its a metal rod that sits inside the flame. Could also be bad and need replaced. Hope this helps.

I assume you have a W-R and should see a small notch down around 5 o'clock or so. That position is where you depress the knob to light the pilot.

most likely you have a thermopile that has low output. it takes about 100mv to maintain pilot light

On your main gas manifold you have orfaces that screw into it at each burner. Remove these and see if their are small spider webs in them. They will look like little lint balls. They can keep your main burner from getting gas and in turn keep the heater from sensing flame. The pilot lights and the heater starts but no flame at the main will shut it rite down. See if this helps.