Hayward natural gas h1001d wont ignite

Not much info to go by here Patty.
A gas fired heater needs spark and fuel to ignite. We need to start by to determining which is missing.
The H series has seen a few progressions. Does yours have no lights, 3 lights, or a lcd display? Propane or natural gas? Standing pilot or spark ignition?
Do any lights or display come on? Do you hear any clicking?

yes, make sure you order the correct part, youll need the model number and serial number. the ignitor has 2 wires wrapped with white fibers, usually 2 screws to remove it, and theres only one way to plug it back in. NOTE: Ignition failure does not always mean the ignitor needs replaced. make sure your ignitor doesnt turn on and glow when you start the heater. if it does, youll want to clean out the gas orifices.

you may need to bleed the line to get fresh gas to the unit, you can use soapy water to check, that the fitting leak free, after bleeding at the block.

The ignitor is not working. I don't know what kind of ignition your heater has but it is either a thermal ignitor or a spark ignitor. In either case the problem could be in the ignitor or the board that supplies the power to the ignotor.

First of all, be careful while looking into the system when the gas comes on because it could unexpectedly ignite and burn you.

If your unit contains an ignitor it should glow brightly when the gas starts. If it is glowing then it is probably not glowing brightly enough or not in the right place and therefore needs to be either replaced or repositioned. If it is not glowing then you should measure the voltage using a volt/ohm meter going to the ignitor. If it reads about 115 then the ignitor is broken and needs to be replaced. If it reads much lower voltage then the board that supplies power to it probably has a problem on it. It may be a defective relay, bad solder joint or some other problem. I would first re-solder the relay connections to the board and see if that fixes it. Otherwise the board will need to be replaced.

If the unit has a spark ignitor and you hear a spark sound it may be too low power or the spark probe might be in the wrong place and therefore not delivering the spark to the fuel air mixture. If you don't hear the spark then you may have a bad relay or bad solder joint to it.

Please let me know how this works out. Thank you.

Make sure you have no soot build up on your ignitor. It will conduct elect so you can hear a buzz instead of a clear spark. Without a clean spark it wont fire and for safty it shuts down to keep gas from building up in the cabnit.

If the fan comes on it has to be the ignitor or the ignition module

Big enough propane tank? proper pressure reducing valve on the propane? Check both of those.