To begin with; you should have a double check valve on lawn sprinklers
to avoid contaminating the house water, most places require this.
If you use chemicals on your lawn, you probably have some of that too.
since the main shut off helped, try that again, & open the nearest
faucet and let it run for like an hour.


My sprinkler valves are anti-siphon. I thought that was all that was
necessary.