DIAGNOSIS: ER1 means the grill's control board isn't detecting a proper signal from one of the knobs or switches. It's a communication error within the electronic control system.
FIX: First, try a full reset. Turn off the main propane tank valve, then disconnect the tank hose. Turn all the burner knobs to OFF. Wait 5 minutes. Reconnect the hose, open the tank valve slowly, and try starting the grill again. This often clears a temporary glitch.
IF THAT DIDN'T WORK: The error likely points to a faulty control board or a bad connection. Check the wiring harness plugs on the back of the control panel to make sure they're fully seated and not corroded.
HEADS UP: Persistent ER1 codes usually require a call to Weber support. The board itself might need replacing, but they can run you through a more detailed diagnostic first.